processdesign - User contributions [en]

Estimation of production cost and revenue

2014-02-25T20:23:49Z

Jxamplas:

==Variable Cost of Production==
Variable costs of production are dependent primarily on plant output and rate of production. There are many variables to consider when costing a plant.
# Raw materials consumed
# Utilities-steam, electricity, cooling water, fuel, etc.
# Consumables - acids, bases, solvents, catalysts, etc.
# Disposal
# Shipping
The majority of the variable costs for a production plant are the raw materials and utilities costs. Variable costs can be greatly cut through optimization techniques and intelligent plant design [1].
===Raw Materials Cost===
Calculating the annual cost of a raw material is calculated by simply multiplying the feed rate of the process by the appropriate price per volume or mass. These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [5].There are several ways to optimize this cost to ensure that a process is not costing more than it should. First one should assess the actual consumption of a plant to see if it is significantly different from what should be expected based on process stoichiometry and selectivities [1]. Finding may prove that a process is less efficient than it originally claimed.
It is smart to benchmark a new plant design against an existing plant or pilot plant. Raw materials are typically the largest contributor to overall variable costs. For bulk chemicals and petrochemicals, raw materials represent 80-90% of the total cash cost of production (CCOP).
===Utilities Cost===
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [5]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

Utility streams are excellent ways to streamline a process and are often indicative of how efficient of a process the project is. Process methods such as steam generation and pinch analysis can be used to greatly reduce utility costs across a plant. Further analysis of pinch analysis techniques and optimizing heat exchanger networks can be found in plant design texts such as first reference from Gavin Towler. The determination of process utility costs is often more difficult than the determination of raw material costs; however, the utilities are typically between 5-10% of CCOP [1]. The cost of heating a process can be reduced by using process waste streams as fuel which consequently also reduces the need for waste disposal.

===Waste Disposal Costs===
These are defined as the cost of waste treatment to protect the environment [5]. These are materials that cannot be recycled or sold off as by-products. Often times these streams require additives or additional treatment to meet governmental regulations.
Hydrocarbon waste can often be incinerated directly to the atmosphere or used as process fuel to heat other streams in the system. Using the stream as process fuel allows the fuel value of the stream to be recovered into the system. The substituted value can be calculated by multiplying the conventional fuel price by the heat of combustion of the waste stream.

<math>P_{WFV} = P_F * \Delta H_C^o</math>

where
<math>P_{WFV}</math> = waste value of fuel ($/lb or $/kg)

<math>P_F</math> = price of fuel ($/MMBtu or $/GJ)

<math>\Delta H_C^o</math> = heat of combustion (MMBtu/lb or GJ/kg)

Dilute aqueous streams must be sent to wastewater treatment typically prior to purging from the plant. Acidic or basic wastes are neutralized prior to treatment by salting out the acid or base. The cost of wastewater treatment is typically about $6 per 1000 gal but this is only an estimate that doesn't account for regional charges [1].

Solid waste treatment can typically be sent to a landfill at a cost of approximately $50/ton [1].

Hazardous wastes arise from the production of concentrated liquid streams that cannot be incinerated. Hazardous wastes should be avoided if possible, but that is not always feasible for some processes. The cost of hazardous waste disposal is strongly dependent on the location of the plant, the plants proximity to waste disposal plants and the degree of hazard of the waste.

==Fixed Cost of Production==
Fixed costs are those whose amounts are independent of production rates. Much of these costs are personnel salaries, taxes, insurance, and legal payments.
===Labor Costs===
These are the costs attributed to the personnel required to operate the process plant [5].

The number of operators required per shift, <math>N_{OL}</math> can be estimated by

<math>N_{OL}=(6.29+31.7P^2+0.23N_{np})^{0.5}</math>

where <math>P</math> is the number of processing steps involving particulate solids and <math>N_{np}</math> is the number of other processing steps [5]. For each of the <math>N_{OL}</math> operators per 8-hour shift, approximately 4.5 operators must be hired for a plant that runs 24 hours per day, to account for the 3 shifts per day and the 3 weeks of leave typically taken by each operator per year [5]. The salary for a chemical plant operator varies by location, and the estimator should look up the average value for the area.

===Maintenance Costs===
These are the costs associated with labor and materials necessary to maintain plant production. An estimate of these are 6% of the fixed capital investment [5].

===Research and Development===
These are the costs of research done in developing the process and/or products. This includes salaries for researchers as well as funds for research related equipment and supplies. An estimate of these costs are 5% of the total manufacturing cost [5].

===Taxes and Insurance===
Taxes vary by location, but a first estimate of property taxes and liability insurance is 3% of the fixed capital investment [5].

===Plant Overhead===
Overhead costs are the miscellaneous but necessary costs of running a business, including payroll, employee benefits, and janitorial services. This may be estimated as 70% of the operating labor costs, added to 4% of the fixed capital costs [5].

===Licensing and Royalties===
The costs of paying for the use of intellectual property clearly varies, but an estimate that may be used is 3% of the total manufacturing cost [5].

==Revenues==
The revenues of a process are the income earned form sales of the main products and the by-products. Revenue can be impacted by market fluctuations and production rates.
===By-Product Revenues===
Besides selling the main product from a process, by-products from separations and reactions can also be valuable in the market. Often it is more difficult to decide which by-products to recover and purify than it is to make decisions on the main product.

By-products made in stoichiometric ratios from reactions must be either sold off or managed through waste disposal. Other by-products are sometimes produced through feed impurities or by nonselective reactions. There are several potential valuable by-products from a process:
# Materials produced in stoichiometric quantities by the reactions that create the main product. If they are not recovered then the waste disposal expenses will be large.
# Components that are produced in high yield by side reactions.
# Components formed in high yield from feed impurities. Many sulfurs are produced as a by-product of fuels manufacture.
# Components that are produced in low yield but have high value. An example includes acetophenone which is recovered as a by-product of phenol manufacture.
# Degraded consumables (e.g. solvents, etc.) that have reuse value.

A rule of thumb that can be used for preliminary screening of by-products for large plants is that for by-product recovery to be economically feasible the net benefit must be greater than $200,000 a year. A net benefit can be calculated by adding the possible resale value of the by-product and the avoided waste disposal cost [1].

===Margin===
The gross margin of a process is defined as the sum of product and by-product revenues minus the raw material cost.

Gross margin = Revenues - Raw materials costs

Because raw materials are most often the most expensive variable cost of a process, the gross margin is a good gauge as to what the total profitability of a process will be. Raw materials and product pricing are often subject to high degrees of variability which can be difficult to forecast. The size of margins are highly versatile depending on the
industry. For many petrochemical industries the margin may be only 10%; however, for industries such as food additives and pharmaceuticals the margins are generally much higher [1].

===Profits===
There are several standards for calculating company profits. The cash cost of production (CCOP) is the sum of the fixed and variable production costs.

<math>CCOP = VCOP + FCOP'</math>

where <math>VCOP</math> is the variable cost of production and <math>FCOP</math> is the fixed cost of production.

Gross profit, which should not be confused with gross margin, is then calculated by the following equation,

<math>Gross\ profit = Main\ product\ revenues - CCOP</math>

Finally profit can be calculated by subtracting the income taxes that the plant would be subject to depending on the tax code of the county the plant is located in.

<math>Net\ profit = gross\ profit - taxes</math>

==Pricing Products and Raw Materials==
The revenues and costs of a project are vital to determining its economic feasibility. To calculate these values one needs to multiply the respective product and feed streams by their respective prices. The major difficulty of this process is determining the prices that should be used in this formula. When analyzing a plant, not only do the current prices need to be acknowledged but also the stability of the market to forecast future fluctuations and deviations.
===Pricing Fundamentals===
The pricing of a substance is determined by the fundamental economic principles of supply and demand. A supply curve and demand curve can be graphed and added to determine the market equilibrium price and projected market size. There are many ways a company can combat if the market equilibrium pricing is not suitable for a process. One of these ways is changing the market that the company is selling to. Instead of selling industrial grade product there may be markets for pharmaceutical grade or food grade that would allow for a company to sell their product at higher margins. Another avenue to look into is changing the geographic market being sold to. Rarely is there a global synchronous market, but rather a variation depending on where in the world the product is being sold. It is possible that a company could make more money by dedicating their sales to the Asian market as opposed to the US or vise versa.
===Price Data Sources===
There are many resources when trying to determine the price of a chemical or utility. This are important for looking at current pricing information as well as historical data that can be used for forecasting purposes.
====Internal Company Forecasts====
Large companies will often have the marketing or development departments develop a forecasting database that can be used internally in the company. Forecasts of this magnitude will often have multiple scenarios and projects that are evaluated under the given parameters. Companies may even license these forecasts to other companies for high fees if they desire.

[[File:Capture.JPG]]

Table 1 provides common industry acronyms that are used to indicate certain key words when determining pricing information.

====Trade Journals====
There are also many publications that report pricing data weekly. ''ICIS Chemical Business Americas'' used to publish the prices for hundreds of chemicals but have more recently changed their data to an online database that requires a subscription. This service is very expensive, but necessary for many companies. ''Oil and Gas Journal'' publishes the market prices of many crude oils and other petrochemicals using data from several continents. This journal also provides margin data for many refineries and plants on a monthly basis. ''Chemical Week'' provides the spot and contract prices for 22 chemicals in the US and European markets.

====Consultants====
If trade journals are not adequate for the information needed, some companies will contract consultants to do deep research into the subject. Consultants are excellent resources for providing economic and marketing information but come at a large price. There are several companies that provide this type of service but some of the larger firms include: ''Purvin and Gertz'', ''Cambidge Energy Research Associates'', ''Chemical Markets Associates Inc.'', and ''SRI: The Chemical Economics Handbook''

====Online Brokers and Suppliers====
Often time price data can be supplied by the supplier themselves and using online directories. Restraint should be used when quoting these prices however because they are often spot prices that are much higher than what would be expected from bulk contract supplying.

==Example Case: Estimating Cost of Production==

Use the following information to estimate the manufacturing cost of a nitric acid plant producing 92,000 tonnes/year.

:Fixed Capital Investment: $11,000,000
:Raw Material Cost: $7,950,000/yr
:Waste Treatment Cost: $1,000,000/yr
:Utilities: $356,000/year
:Direct Labor Cost: $300,000/yr
:Fixed Costs: $1,500,000/yr

To find the manufacturing cost, multiply the value of the variable cost by the appropriate cost factor. In this case, we get,

<math>COM_d = (0.180)($11,000,000) + (2.73)($300,000) + (1.23($356,000 + $1,000,000 + $7,950,000) = $14,245,000/yr</math>

<math>($14,245,000/yr)/(92,000\ tonne/yr) = $155/tonne</math>

==Conclusion==

==References==
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-25T20:23:15Z

Jxamplas: /* Profits */

==Variable Cost of Production==
Variable costs of production are dependent primarily on plant output and rate of production. There are many variables to consider when costing a plant.
# Raw materials consumed
# Utilities-steam, electricity, cooling water, fuel, etc.
# Consumables - acids, bases, solvents, catalysts, etc.
# Disposal
# Shipping
The majority of the variable costs for a production plant are the raw materials and utilities costs. Variable costs can be greatly cut through optimization techniques and intelligent plant design [1].
===Raw Materials Cost===
Calculating the annual cost of a raw material is calculated by simply multiplying the feed rate of the process by the appropriate price per volume or mass. These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [5].There are several ways to optimize this cost to ensure that a process is not costing more than it should. First one should assess the actual consumption of a plant to see if it is significantly different from what should be expected based on process stoichiometry and selectivities [1]. Finding may prove that a process is less efficient than it originally claimed.
It is smart to benchmark a new plant design against an existing plant or pilot plant. Raw materials are typically the largest contributor to overall variable costs. For bulk chemicals and petrochemicals, raw materials represent 80-90% of the total cash cost of production (CCOP).
===Utilities Cost===
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [5]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

Utility streams are excellent ways to streamline a process and are often indicative of how efficient of a process the project is. Process methods such as steam generation and pinch analysis can be used to greatly reduce utility costs across a plant. Further analysis of pinch analysis techniques and optimizing heat exchanger networks can be found in plant design texts such as first reference from Gavin Towler. The determination of process utility costs is often more difficult than the determination of raw material costs; however, the utilities are typically between 5-10% of CCOP [1]. The cost of heating a process can be reduced by using process waste streams as fuel which consequently also reduces the need for waste disposal.

===Waste Disposal Costs===
These are defined as the cost of waste treatment to protect the environment [5]. These are materials that cannot be recycled or sold off as by-products. Often times these streams require additives or additional treatment to meet governmental regulations.
Hydrocarbon waste can often be incinerated directly to the atmosphere or used as process fuel to heat other streams in the system. Using the stream as process fuel allows the fuel value of the stream to be recovered into the system. The substituted value can be calculated by multiplying the conventional fuel price by the heat of combustion of the waste stream.

<math>P_{WFV} = P_F * \Delta H_C^o</math>

where
<math>P_{WFV}</math> = waste value of fuel ($/lb or $/kg)

<math>P_F</math> = price of fuel ($/MMBtu or $/GJ)

<math>\Delta H_C^o</math> = heat of combustion (MMBtu/lb or GJ/kg)

Dilute aqueous streams must be sent to wastewater treatment typically prior to purging from the plant. Acidic or basic wastes are neutralized prior to treatment by salting out the acid or base. The cost of wastewater treatment is typically about $6 per 1000 gal but this is only an estimate that doesn't account for regional charges [1].

Solid waste treatment can typically be sent to a landfill at a cost of approximately $50/ton [1].

Hazardous wastes arise from the production of concentrated liquid streams that cannot be incinerated. Hazardous wastes should be avoided if possible, but that is not always feasible for some processes. The cost of hazardous waste disposal is strongly dependent on the location of the plant, the plants proximity to waste disposal plants and the degree of hazard of the waste.

==Fixed Cost of Production==
Fixed costs are those whose amounts are independent of production rates. Much of these costs are personnel salaries, taxes, insurance, and legal payments.
===Labor Costs===
These are the costs attributed to the personnel required to operate the process plant [5].

The number of operators required per shift, <math>N_{OL}</math> can be estimated by

<math>N_{OL}=(6.29+31.7P^2+0.23N_{np})^{0.5}</math>

where <math>P</math> is the number of processing steps involving particulate solids and <math>N_{np}</math> is the number of other processing steps [5]. For each of the <math>N_{OL}</math> operators per 8-hour shift, approximately 4.5 operators must be hired for a plant that runs 24 hours per day, to account for the 3 shifts per day and the 3 weeks of leave typically taken by each operator per year [5]. The salary for a chemical plant operator varies by location, and the estimator should look up the average value for the area.

===Maintenance Costs===
These are the costs associated with labor and materials necessary to maintain plant production. An estimate of these are 6% of the fixed capital investment [5].

===Research and Development===
These are the costs of research done in developing the process and/or products. This includes salaries for researchers as well as funds for research related equipment and supplies. An estimate of these costs are 5% of the total manufacturing cost [5].

===Taxes and Insurance===
Taxes vary by location, but a first estimate of property taxes and liability insurance is 3% of the fixed capital investment [5].

===Plant Overhead===
Overhead costs are the miscellaneous but necessary costs of running a business, including payroll, employee benefits, and janitorial services. This may be estimated as 70% of the operating labor costs, added to 4% of the fixed capital costs [5].

===Licensing and Royalties===
The costs of paying for the use of intellectual property clearly varies, but an estimate that may be used is 3% of the total manufacturing cost [5].

==Revenues==
The revenues of a process are the income earned form sales of the main products and the by-products. Revenue can be impacted by market fluctuations and production rates.
===By-Product Revenues===
Besides selling the main product from a process, by-products from separations and reactions can also be valuable in the market. Often it is more difficult to decide which by-products to recover and purify than it is to make decisions on the main product.

By-products made in stoichiometric ratios from reactions must be either sold off or managed through waste disposal. Other by-products are sometimes produced through feed impurities or by nonselective reactions. There are several potential valuable by-products from a process:
# Materials produced in stoichiometric quantities by the reactions that create the main product. If they are not recovered then the waste disposal expenses will be large.
# Components that are produced in high yield by side reactions.
# Components formed in high yield from feed impurities. Many sulfurs are produced as a by-product of fuels manufacture.
# Components that are produced in low yield but have high value. An example includes acetophenone which is recovered as a by-product of phenol manufacture.
# Degraded consumables (e.g. solvents, etc.) that have reuse value.

A rule of thumb that can be used for preliminary screening of by-products for large plants is that for by-product recovery to be economically feasible the net benefit must be greater than $200,000 a year. A net benefit can be calculated by adding the possible resale value of the by-product and the avoided waste disposal cost [1].

===Margin===
The gross margin of a process is defined as the sum of product and by-product revenues minus the raw material cost.

Gross margin = Revenues - Raw materials costs

Because raw materials are most often the most expensive variable cost of a process, the gross margin is a good gauge as to what the total profitability of a process will be. Raw materials and product pricing are often subject to high degrees of variability which can be difficult to forecast. The size of margins are highly versatile depending on the
industry. For many petrochemical industries the margin may be only 10%; however, for industries such as food additives and pharmaceuticals the margins are generally much higher [1].

===Profits===
There are several standards for calculating company profits. The cash cost of production (CCOP) is the sum of the fixed and variable production costs.

<math>CCOP = VCOP + FCOP'</math>

where <math>VCOP</math> is the variable cost of production and <math>FCOP</math> is the fixed cost of production.

Gross profit, which should not be confused with gross margin, is then calculated by the following equation,

<math>Gross\ profit = Main\ product\ revenues - CCOP</math>

Finally profit can be calculated by subtracting the income taxes that the plant would be subject to depending on the tax code of the county the plant is located in.

<math>Net\ profit = gross\ profit - taxes</math>

==Pricing Products and Raw Materials==
The revenues and costs of a project are vital to determining its economic feasibility. To calculate these values one needs to multiply the respective product and feed streams by their respective prices. The major difficulty of this process is determining the prices that should be used in this formula. When analyzing a plant, not only do the current prices need to be acknowledged but also the stability of the market to forecast future fluctuations and deviations.
===Pricing Fundamentals===
The pricing of a substance is determined by the fundamental economic principles of supply and demand. A supply curve and demand curve can be graphed and added to determine the market equilibrium price and projected market size. There are many ways a company can combat if the market equilibrium pricing is not suitable for a process. One of these ways is changing the market that the company is selling to. Instead of selling industrial grade product there may be markets for pharmaceutical grade or food grade that would allow for a company to sell their product at higher margins. Another avenue to look into is changing the geographic market being sold to. Rarely is there a global synchronous market, but rather a variation depending on where in the world the product is being sold. It is possible that a company could make more money by dedicating their sales to the Asian market as opposed to the US or vise versa.
===Price Data Sources===
There are many resources when trying to determine the price of a chemical or utility. This are important for looking at current pricing information as well as historical data that can be used for forecasting purposes.
====Internal Company Forecasts====
Large companies will often have the marketing or development departments develop a forecasting database that can be used internally in the company. Forecasts of this magnitude will often have multiple scenarios and projects that are evaluated under the given parameters. Companies may even license these forecasts to other companies for high fees if they desire.

[[File:Capture.JPG]]

Table 1 provides common industry acronyms that are used to indicate certain key words when determining pricing information.

====Trade Journals====
There are also many publications that report pricing data weekly. ''ICIS Chemical Business Americas'' used to publish the prices for hundreds of chemicals but have more recently changed their data to an online database that requires a subscription. This service is very expensive, but necessary for many companies. ''Oil and Gas Journal'' publishes the market prices of many crude oils and other petrochemicals using data from several continents. This journal also provides margin data for many refineries and plants on a monthly basis. ''Chemical Week'' provides the spot and contract prices for 22 chemicals in the US and European markets.

====Consultants====
If trade journals are not adequate for the information needed, some companies will contract consultants to do deep research into the subject. Consultants are excellent resources for providing economic and marketing information but come at a large price. There are several companies that provide this type of service but some of the larger firms include: ''Purvin and Gertz'', ''Cambidge Energy Research Associates'', ''Chemical Markets Associates Inc.'', and ''SRI: The Chemical Economics Handbook''

====Online Brokers and Suppliers====
Often time price data can be supplied by the supplier themselves and using online directories. Restraint should be used when quoting these prices however because they are often spot prices that are much higher than what would be expected from bulk contract supplying.

==Example Case: Estimating Cost of Production==

Use the following information to estimate the manufacturing cost of a nitric acid plant producing 92,000 tonnes/year.

:Fixed Capital Investment: $11,000,000
:Raw Material Cost: $7,950,000/yr
:Waste Treatment Cost: $1,000,000/yr
:Utilities: $356,000/year
:Direct Labor Cost: $300,000/yr
:Fixed Costs: $1,500,000/yr

To find the manufacturing cost, multiply the value of the variable cost by the appropriate cost factor. In this case, we get,

<math>COM_d = (0.180)($11,000,000) + (2.73)($300,000) + (1.23($356,000 + $1,000,000 + $7,950,000) = $14,245,000/yr</math>

<math>($14,245,000/yr)/(92,000\ tonne/yr) = $155/tonne</math>

==References==
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-25T20:22:42Z

Jxamplas: /* Profits */

==Variable Cost of Production==
Variable costs of production are dependent primarily on plant output and rate of production. There are many variables to consider when costing a plant.
# Raw materials consumed
# Utilities-steam, electricity, cooling water, fuel, etc.
# Consumables - acids, bases, solvents, catalysts, etc.
# Disposal
# Shipping
The majority of the variable costs for a production plant are the raw materials and utilities costs. Variable costs can be greatly cut through optimization techniques and intelligent plant design [1].
===Raw Materials Cost===
Calculating the annual cost of a raw material is calculated by simply multiplying the feed rate of the process by the appropriate price per volume or mass. These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [5].There are several ways to optimize this cost to ensure that a process is not costing more than it should. First one should assess the actual consumption of a plant to see if it is significantly different from what should be expected based on process stoichiometry and selectivities [1]. Finding may prove that a process is less efficient than it originally claimed.
It is smart to benchmark a new plant design against an existing plant or pilot plant. Raw materials are typically the largest contributor to overall variable costs. For bulk chemicals and petrochemicals, raw materials represent 80-90% of the total cash cost of production (CCOP).
===Utilities Cost===
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [5]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

Utility streams are excellent ways to streamline a process and are often indicative of how efficient of a process the project is. Process methods such as steam generation and pinch analysis can be used to greatly reduce utility costs across a plant. Further analysis of pinch analysis techniques and optimizing heat exchanger networks can be found in plant design texts such as first reference from Gavin Towler. The determination of process utility costs is often more difficult than the determination of raw material costs; however, the utilities are typically between 5-10% of CCOP [1]. The cost of heating a process can be reduced by using process waste streams as fuel which consequently also reduces the need for waste disposal.

===Waste Disposal Costs===
These are defined as the cost of waste treatment to protect the environment [5]. These are materials that cannot be recycled or sold off as by-products. Often times these streams require additives or additional treatment to meet governmental regulations.
Hydrocarbon waste can often be incinerated directly to the atmosphere or used as process fuel to heat other streams in the system. Using the stream as process fuel allows the fuel value of the stream to be recovered into the system. The substituted value can be calculated by multiplying the conventional fuel price by the heat of combustion of the waste stream.

<math>P_{WFV} = P_F * \Delta H_C^o</math>

where
<math>P_{WFV}</math> = waste value of fuel ($/lb or $/kg)

<math>P_F</math> = price of fuel ($/MMBtu or $/GJ)

<math>\Delta H_C^o</math> = heat of combustion (MMBtu/lb or GJ/kg)

Dilute aqueous streams must be sent to wastewater treatment typically prior to purging from the plant. Acidic or basic wastes are neutralized prior to treatment by salting out the acid or base. The cost of wastewater treatment is typically about $6 per 1000 gal but this is only an estimate that doesn't account for regional charges [1].

Solid waste treatment can typically be sent to a landfill at a cost of approximately $50/ton [1].

Hazardous wastes arise from the production of concentrated liquid streams that cannot be incinerated. Hazardous wastes should be avoided if possible, but that is not always feasible for some processes. The cost of hazardous waste disposal is strongly dependent on the location of the plant, the plants proximity to waste disposal plants and the degree of hazard of the waste.

==Fixed Cost of Production==
Fixed costs are those whose amounts are independent of production rates. Much of these costs are personnel salaries, taxes, insurance, and legal payments.
===Labor Costs===
These are the costs attributed to the personnel required to operate the process plant [5].

The number of operators required per shift, <math>N_{OL}</math> can be estimated by

<math>N_{OL}=(6.29+31.7P^2+0.23N_{np})^{0.5}</math>

where <math>P</math> is the number of processing steps involving particulate solids and <math>N_{np}</math> is the number of other processing steps [5]. For each of the <math>N_{OL}</math> operators per 8-hour shift, approximately 4.5 operators must be hired for a plant that runs 24 hours per day, to account for the 3 shifts per day and the 3 weeks of leave typically taken by each operator per year [5]. The salary for a chemical plant operator varies by location, and the estimator should look up the average value for the area.

===Maintenance Costs===
These are the costs associated with labor and materials necessary to maintain plant production. An estimate of these are 6% of the fixed capital investment [5].

===Research and Development===
These are the costs of research done in developing the process and/or products. This includes salaries for researchers as well as funds for research related equipment and supplies. An estimate of these costs are 5% of the total manufacturing cost [5].

===Taxes and Insurance===
Taxes vary by location, but a first estimate of property taxes and liability insurance is 3% of the fixed capital investment [5].

===Plant Overhead===
Overhead costs are the miscellaneous but necessary costs of running a business, including payroll, employee benefits, and janitorial services. This may be estimated as 70% of the operating labor costs, added to 4% of the fixed capital costs [5].

===Licensing and Royalties===
The costs of paying for the use of intellectual property clearly varies, but an estimate that may be used is 3% of the total manufacturing cost [5].

==Revenues==
The revenues of a process are the income earned form sales of the main products and the by-products. Revenue can be impacted by market fluctuations and production rates.
===By-Product Revenues===
Besides selling the main product from a process, by-products from separations and reactions can also be valuable in the market. Often it is more difficult to decide which by-products to recover and purify than it is to make decisions on the main product.

By-products made in stoichiometric ratios from reactions must be either sold off or managed through waste disposal. Other by-products are sometimes produced through feed impurities or by nonselective reactions. There are several potential valuable by-products from a process:
# Materials produced in stoichiometric quantities by the reactions that create the main product. If they are not recovered then the waste disposal expenses will be large.
# Components that are produced in high yield by side reactions.
# Components formed in high yield from feed impurities. Many sulfurs are produced as a by-product of fuels manufacture.
# Components that are produced in low yield but have high value. An example includes acetophenone which is recovered as a by-product of phenol manufacture.
# Degraded consumables (e.g. solvents, etc.) that have reuse value.

A rule of thumb that can be used for preliminary screening of by-products for large plants is that for by-product recovery to be economically feasible the net benefit must be greater than $200,000 a year. A net benefit can be calculated by adding the possible resale value of the by-product and the avoided waste disposal cost [1].

===Margin===
The gross margin of a process is defined as the sum of product and by-product revenues minus the raw material cost.

Gross margin = Revenues - Raw materials costs

Because raw materials are most often the most expensive variable cost of a process, the gross margin is a good gauge as to what the total profitability of a process will be. Raw materials and product pricing are often subject to high degrees of variability which can be difficult to forecast. The size of margins are highly versatile depending on the
industry. For many petrochemical industries the margin may be only 10%; however, for industries such as food additives and pharmaceuticals the margins are generally much higher [1].

===Profits===
There are several standards for calculating company profits. The cash cost of production (CCOP) is the sum of the fixed and variable production costs.

<math>CCOP = VCOP + FCOP'</math>

where <math>VCOP</math> is the variable cost of production and <math>FCOP</math> is the fixed cost of production.

Gross profit, which should not be confused with gross margin, is then calculated by the following equation,

<math>Gross profit = Main product revenues - CCOP</math>

Finally profit can be calculated by subtracting the income taxes that the plant would be subject to depending on the tax code of the county the plant is located in.

<math>Net profit = gross profit - taxes</math>

==Pricing Products and Raw Materials==
The revenues and costs of a project are vital to determining its economic feasibility. To calculate these values one needs to multiply the respective product and feed streams by their respective prices. The major difficulty of this process is determining the prices that should be used in this formula. When analyzing a plant, not only do the current prices need to be acknowledged but also the stability of the market to forecast future fluctuations and deviations.
===Pricing Fundamentals===
The pricing of a substance is determined by the fundamental economic principles of supply and demand. A supply curve and demand curve can be graphed and added to determine the market equilibrium price and projected market size. There are many ways a company can combat if the market equilibrium pricing is not suitable for a process. One of these ways is changing the market that the company is selling to. Instead of selling industrial grade product there may be markets for pharmaceutical grade or food grade that would allow for a company to sell their product at higher margins. Another avenue to look into is changing the geographic market being sold to. Rarely is there a global synchronous market, but rather a variation depending on where in the world the product is being sold. It is possible that a company could make more money by dedicating their sales to the Asian market as opposed to the US or vise versa.
===Price Data Sources===
There are many resources when trying to determine the price of a chemical or utility. This are important for looking at current pricing information as well as historical data that can be used for forecasting purposes.
====Internal Company Forecasts====
Large companies will often have the marketing or development departments develop a forecasting database that can be used internally in the company. Forecasts of this magnitude will often have multiple scenarios and projects that are evaluated under the given parameters. Companies may even license these forecasts to other companies for high fees if they desire.

[[File:Capture.JPG]]

Table 1 provides common industry acronyms that are used to indicate certain key words when determining pricing information.

====Trade Journals====
There are also many publications that report pricing data weekly. ''ICIS Chemical Business Americas'' used to publish the prices for hundreds of chemicals but have more recently changed their data to an online database that requires a subscription. This service is very expensive, but necessary for many companies. ''Oil and Gas Journal'' publishes the market prices of many crude oils and other petrochemicals using data from several continents. This journal also provides margin data for many refineries and plants on a monthly basis. ''Chemical Week'' provides the spot and contract prices for 22 chemicals in the US and European markets.

====Consultants====
If trade journals are not adequate for the information needed, some companies will contract consultants to do deep research into the subject. Consultants are excellent resources for providing economic and marketing information but come at a large price. There are several companies that provide this type of service but some of the larger firms include: ''Purvin and Gertz'', ''Cambidge Energy Research Associates'', ''Chemical Markets Associates Inc.'', and ''SRI: The Chemical Economics Handbook''

====Online Brokers and Suppliers====
Often time price data can be supplied by the supplier themselves and using online directories. Restraint should be used when quoting these prices however because they are often spot prices that are much higher than what would be expected from bulk contract supplying.

==Example Case: Estimating Cost of Production==

Use the following information to estimate the manufacturing cost of a nitric acid plant producing 92,000 tonnes/year.

:Fixed Capital Investment: $11,000,000
:Raw Material Cost: $7,950,000/yr
:Waste Treatment Cost: $1,000,000/yr
:Utilities: $356,000/year
:Direct Labor Cost: $300,000/yr
:Fixed Costs: $1,500,000/yr

To find the manufacturing cost, multiply the value of the variable cost by the appropriate cost factor. In this case, we get,

<math>COM_d = (0.180)($11,000,000) + (2.73)($300,000) + (1.23($356,000 + $1,000,000 + $7,950,000) = $14,245,000/yr</math>

<math>($14,245,000/yr)/(92,000\ tonne/yr) = $155/tonne</math>

==References==
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-24T05:52:18Z

Jxamplas: /* Example Case: Estimating Cost of Production */

==Variable Cost of Production==
Variable costs of production are dependent primarily on plant output and rate of production. There are many variables to consider when costing a plant.
# Raw materials consumed
# Utilities-steam, electricity, cooling water, fuel, etc.
# Consumables - acids, bases, solvents, catalysts, etc.
# Disposal
# Shipping
The majority of the variable costs for a production plant are the raw materials and utilities costs. Variable costs can be greatly cut through optimization techniques and intelligent plant design [1].
===Raw Materials Cost===
Calculating the annual cost of a raw material is calculated by simply multiplying the feed rate of the process by the appropriate price per volume or mass. These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [5].There are several ways to optimize this cost to ensure that a process is not costing more than it should. First one should assess the actual consumption of a plant to see if it is significantly different from what should be expected based on process stoichiometry and selectivities [1]. Finding may prove that a process is less efficient than it originally claimed.
It is smart to benchmark a new plant design against an existing plant or pilot plant. Raw materials are typically the largest contributor to overall variable costs. For bulk chemicals and petrochemicals, raw materials represent 80-90% of the total cash cost of production (CCOP).
===Utilities Cost===
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [5]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

Utility streams are excellent ways to streamline a process and are often indicative of how efficient of a process the project is. Process methods such as steam generation and pinch analysis can be used to greatly reduce utility costs across a plant. Further analysis of pinch analysis techniques and optimizing heat exchanger networks can be found in plant design texts such as first reference from Gavin Towler. The determination of process utility costs is often more difficult than the determination of raw material costs; however, the utilities are typically between 5-10% of CCOP [1]. The cost of heating a process can be reduced by using process waste streams as fuel which consequently also reduces the need for waste disposal.

===Waste Disposal Costs===
These are defined as the cost of waste treatment to protect the environment [5]. These are materials that cannot be recycled or sold off as by-products. Often times these streams require additives or additional treatment to meet governmental regulations.
Hydrocarbon waste can often be incinerated directly to the atmosphere or used as process fuel to heat other streams in the system. Using the stream as process fuel allows the fuel value of the stream to be recovered into the system. The substituted value can be calculated by multiplying the conventional fuel price by the heat of combustion of the waste stream.

<math>P_{WFV} = P_F * \Delta H_C^o</math>

where
<math>P_{WFV}</math> = waste value of fuel ($/lb or $/kg)

<math>P_F</math> = price of fuel ($/MMBtu or $/GJ)

<math>\Delta H_C^o</math> = heat of combustion (MMBtu/lb or GJ/kg)

Dilute aqueous streams must be sent to wastewater treatment typically prior to purging from the plant. Acidic or basic wastes are neutralized prior to treatment by salting out the acid or base. The cost of wastewater treatment is typically about $6 per 1000 gal but this is only an estimate that doesn't account for regional charges [1].

Solid waste treatment can typically be sent to a landfill at a cost of approximately $50/ton [1].

Hazardous wastes arise from the production of concentrated liquid streams that cannot be incinerated. Hazardous wastes should be avoided if possible, but that is not always feasible for some processes. The cost of hazardous waste disposal is strongly dependent on the location of the plant, the plants proximity to waste disposal plants and the degree of hazard of the waste.

==Fixed Cost of Production==
Fixed costs are those whose amounts are independent of production rates. Much of these costs are personnel salaries, rent, taxes, insurance, and legal payments.
===Labor Costs===
These are the costs attributed to the personnel required to operate the process plant [5].

The number of operators required per shift, <math>N_{OL}</math> can be calculated by

<math>N_{OL}=(6.29+31.7P^2+0.23N_{np})^{0.5}</math>

where <math>P</math> is the number of processing steps involving particulate solids and <math>N_{np}</math> is the number of other processing steps.[5] For each of the <math>N_{OL}</math> operators per 8-hour shift, approximately 4.5 operators must be hired for a plant that runs 24 hours per day, to account for the 3 shifts per day and the 3 weeks of leave typically taken by each operator per year.[5] The salary for a chemical plant operator varies by location, and the estimator should look up the average value for the area.

===Maintenance Costs===
Part of these costs are associated with labor and materials necessary to maintain plant production [5].

===Research and Development===
These are the costs of research done in developing the process and/or products. This includes salaries for researchers as well as funds for research related equipment and supplies [5].

===Land, Rent, Taxes===
===Insurance===
===Interest Payments===
===Corporate Overhead===
===Licensing and Royalties===

==Revenues==
The revenues of a process are the income earned form sales of the main products and the by-products. Revenue can be impacted by market fluctuations and production rates.
===By-Product Revenues===
Besides selling the main product from a process, by-products from separations and reactions can also be valuable in the market. Often it is more difficult to decide which by-products to recover and purify than it is to make decisions on the main product.

By-products made in stoichiometric ratios from reactions must be either sold off or managed through waste disposal. Other by-products are sometimes produced through feed impurities or by nonselective reactions. There are several potential valuable by-products from a process:
# Materials produced in stoichiometric quantities by the reactions that create the main product. If they are not recovered then the waste disposal expenses will be large.
# Components that are produced in high yield by side reactions.
# Components formed in high yield from feed impurities. Many sulfurs are produced as a by-product of fuels manufacture.
# Components that are produced in low yield but have high value. An example includes acetophenone which is recovered as a by-product of phenol manufacture.
# Degraded consumables (e.g. solvents, etc.) that have reuse value.

A rule of thumb that can be used for preliminary screening of by-products for large plants is that for by-product recovery to be economically feasible the net benefit must be greater than $200,000 a year. A net benefit can be calculated by adding the possible resale value of the by-product and the avoided waste disposal cost [1].

===Margin===
The gross margin of a process is defined as the sum of product and by-product revenues minus the raw material cost.

Gross margin = Revenues - Raw materials costs

Because raw materials are most often the most expensive variable cost of a process, the gross margin is a good gauge as to what the total profitability of a process will be. Raw materials and product pricing are often subject to high degrees of variability which can be difficult to forecast. The size of margins are highly versatile depending on the
industry. For many petrochemical industries the margin may be only 10%; however, for industries such as food additives and pharmaceuticals the margins are generally much higher [1].

===Profits===
There are several standards for calculating company profits. The cash cost of production (CCOP) is the sum of the fixed and variable production costs.

''CCOP = VCOP + FCOP''

where VCOP is the variable cost of production and FCOP is the fixed cost of production.

Gross profit, which should not be confused with gross margin, is then calculated by the following equation,

''Gross profit = Main product revenues - CCOP''

Finally profit can be calculated by subtracting the income taxes that the plant would be subject to depending on the tax code of the county the plant is located in.

''Net profit = gross profit - taxes''

==Pricing Products and Raw Materials==
The revenues and costs of a project are vital to determining its economic feasibility. To calculate these values one needs to multiply the respective product and feed streams by their respective prices. The major difficulty of this process is determining the prices that should be used in this formula. When analyzing a plant, not only do the current prices need to be acknowledged but also the stability of the market to forecast future fluctuations and deviations.
===Pricing Fundamentals===
The pricing of a substance is determined by the fundamental economic principles of supply and demand. A supply curve and demand curve can be graphed and added to determine the market equilibrium price and projected market size. There are many ways a company can combat if the market equilibrium pricing is not suitable for a process. One of these ways is changing the market that the company is selling to. Instead of selling industrial grade product there may be markets for pharmaceutical grade or food grade that would allow for a company to sell their product at higher margins. Another avenue to look into is changing the geographic market being sold to. Rarely is there a global synchronous market, but rather a variation depending on where in the world the product is being sold. It is possible that a company could make more money by dedicating their sales to the Asian market as opposed to the US or vise versa.
===Price Data Sources===
There are many resources when trying to determine the price of a chemical or utility. This are important for looking at current pricing information as well as historical data that can be used for forecasting purposes.
====Internal Company Forecasts====
Large companies will often have the marketing or development departments develop a forecasting database that can be used internally in the company. Forecasts of this magnitude will often have multiple scenarios and projects that are evaluated under the given parameters. Companies may even license these forecasts to other companies for high fees if they desire.

[[File:Capture.JPG]]

Table 1 provides common industry acronyms that are used to indicate certain key words when determining pricing information.

====Trade Journals====
There are also many publications that report pricing data weekly. ''ICIS Chemical Business Americas'' used to publish the prices for hundreds of chemicals but have more recently changed their data to an online database that requires a subscription. This service is very expensive, but necessary for many companies. ''Oil and Gas Journal'' publishes the market prices of many crude oils and other petrochemicals using data from several continents. This journal also provides margin data for many refineries and plants on a monthly basis. ''Chemical Week'' provides the spot and contract prices for 22 chemicals in the US and European markets.

====Consultants====
If trade journals are not adequate for the information needed, some companies will contract consultants to do deep research into the subject. Consultants are excellent resources for providing economic and marketing information but come at a large price. There are several companies that provide this type of service but some of the larger firms include: ''Purvin and Gertz'', ''Cambidge Energy Research Associates'', ''Chemical Markets Associates Inc.'', and ''SRI: The Chemical Economics Handbook''

====Online Brokers and Suppliers====
Often time price data can be supplied by the supplier themselves and using online directories. Restraint should be used when quoting these prices however because they are often spot prices that are much higher than what would be expected from bulk contract supplying.

==Example Case: Estimating Cost of Production==

Use the following information to estimate the manufacturing cost of a nitric acid plant producing 92,000 tonnes/year.

:Fixed Capital Investment: $11,000,000
:Raw Material Cost: $7,950,000/yr
:Waste Treatment Cost: $1,000,000/yr
:Utilities: $356,000/year
:Direct Labor Cost: $300,000/yr
:Fixed Costs: $1,500,000/yr

To find the manufacturing cost, multiply the value of the variable cost by the appropriate cost factor. In this case, we get,

<math>COM_d = (0.180)($11,000,000) + (2.73)($300,000) + (1.23($356,000 + $1,000,000 + $7,950,000) = $14,245,000/yr</math>

<math>($14,245,000/yr)/(92,000\ tonne/yr) = $155/tonne</math>

==References==
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-24T05:49:06Z

Jxamplas: /* Example Case: Estimating Cost of Production */

==Variable Cost of Production==
Variable costs of production are dependent primarily on plant output and rate of production. There are many variables to consider when costing a plant.
# Raw materials consumed
# Utilities-steam, electricity, cooling water, fuel, etc.
# Consumables - acids, bases, solvents, catalysts, etc.
# Disposal
# Shipping
The majority of the variable costs for a production plant are the raw materials and utilities costs. Variable costs can be greatly cut through optimization techniques and intelligent plant design [1].
===Raw Materials Cost===
Calculating the annual cost of a raw material is calculated by simply multiplying the feed rate of the process by the appropriate price per volume or mass. These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [5].There are several ways to optimize this cost to ensure that a process is not costing more than it should. First one should assess the actual consumption of a plant to see if it is significantly different from what should be expected based on process stoichiometry and selectivities [1]. Finding may prove that a process is less efficient than it originally claimed.
It is smart to benchmark a new plant design against an existing plant or pilot plant. Raw materials are typically the largest contributor to overall variable costs. For bulk chemicals and petrochemicals, raw materials represent 80-90% of the total cash cost of production (CCOP).
===Utilities Cost===
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [5]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

Utility streams are excellent ways to streamline a process and are often indicative of how efficient of a process the project is. Process methods such as steam generation and pinch analysis can be used to greatly reduce utility costs across a plant. Further analysis of pinch analysis techniques and optimizing heat exchanger networks can be found in plant design texts such as first reference from Gavin Towler. The determination of process utility costs is often more difficult than the determination of raw material costs; however, the utilities are typically between 5-10% of CCOP [1]. The cost of heating a process can be reduced by using process waste streams as fuel which consequently also reduces the need for waste disposal.

===Waste Disposal Costs===
These are defined as the cost of waste treatment to protect the environment [5]. These are materials that cannot be recycled or sold off as by-products. Often times these streams require additives or additional treatment to meet governmental regulations.
Hydrocarbon waste can often be incinerated directly to the atmosphere or used as process fuel to heat other streams in the system. Using the stream as process fuel allows the fuel value of the stream to be recovered into the system. The substituted value can be calculated by multiplying the conventional fuel price by the heat of combustion of the waste stream.

<math>P_{WFV} = P_F * \Delta H_C^o</math>

where
<math>P_{WFV}</math> = waste value of fuel ($/lb or $/kg)

<math>P_F</math> = price of fuel ($/MMBtu or $/GJ)

<math>\Delta H_C^o</math> = heat of combustion (MMBtu/lb or GJ/kg)

Dilute aqueous streams must be sent to wastewater treatment typically prior to purging from the plant. Acidic or basic wastes are neutralized prior to treatment by salting out the acid or base. The cost of wastewater treatment is typically about $6 per 1000 gal but this is only an estimate that doesn't account for regional charges [1].

Solid waste treatment can typically be sent to a landfill at a cost of approximately $50/ton [1].

Hazardous wastes arise from the production of concentrated liquid streams that cannot be incinerated. Hazardous wastes should be avoided if possible, but that is not always feasible for some processes. The cost of hazardous waste disposal is strongly dependent on the location of the plant, the plants proximity to waste disposal plants and the degree of hazard of the waste.

==Fixed Cost of Production==
Fixed costs are those whose amounts are independent of production rates. Much of these costs are personnel salaries, rent, taxes, insurance, and legal payments.
===Labor Costs===
These are the costs attributed to the personnel required to operate the process plant [5].

The number of operators required per shift, <math>N_{OL}</math> can be calculated by

<math>N_{OL}=(6.29+31.7P^2+0.23N_{np})^{0.5}</math>

where <math>P</math> is the number of processing steps involving particulate solids and <math>N_{np}</math> is the number of other processing steps.[5] For each of the <math>N_{OL}</math> operators per 8-hour shift, approximately 4.5 operators must be hired for a plant that runs 24 hours per day, to account for the 3 shifts per day and the 3 weeks of leave typically taken by each operator per year.[5] The salary for a chemical plant operator varies by location, and the estimator should look up the average value for the area.

===Maintenance Costs===
Part of these costs are associated with labor and materials necessary to maintain plant production [5].

===Research and Development===
These are the costs of research done in developing the process and/or products. This includes salaries for researchers as well as funds for research related equipment and supplies [5].

===Land, Rent, Taxes===
===Insurance===
===Interest Payments===
===Corporate Overhead===
===Licensing and Royalties===

==Revenues==
The revenues of a process are the income earned form sales of the main products and the by-products. Revenue can be impacted by market fluctuations and production rates.
===By-Product Revenues===
Besides selling the main product from a process, by-products from separations and reactions can also be valuable in the market. Often it is more difficult to decide which by-products to recover and purify than it is to make decisions on the main product.

By-products made in stoichiometric ratios from reactions must be either sold off or managed through waste disposal. Other by-products are sometimes produced through feed impurities or by nonselective reactions. There are several potential valuable by-products from a process:
# Materials produced in stoichiometric quantities by the reactions that create the main product. If they are not recovered then the waste disposal expenses will be large.
# Components that are produced in high yield by side reactions.
# Components formed in high yield from feed impurities. Many sulfurs are produced as a by-product of fuels manufacture.
# Components that are produced in low yield but have high value. An example includes acetophenone which is recovered as a by-product of phenol manufacture.
# Degraded consumables (e.g. solvents, etc.) that have reuse value.

A rule of thumb that can be used for preliminary screening of by-products for large plants is that for by-product recovery to be economically feasible the net benefit must be greater than $200,000 a year. A net benefit can be calculated by adding the possible resale value of the by-product and the avoided waste disposal cost [1].

===Margin===
The gross margin of a process is defined as the sum of product and by-product revenues minus the raw material cost.

Gross margin = Revenues - Raw materials costs

Because raw materials are most often the most expensive variable cost of a process, the gross margin is a good gauge as to what the total profitability of a process will be. Raw materials and product pricing are often subject to high degrees of variability which can be difficult to forecast. The size of margins are highly versatile depending on the
industry. For many petrochemical industries the margin may be only 10%; however, for industries such as food additives and pharmaceuticals the margins are generally much higher [1].

===Profits===
There are several standards for calculating company profits. The cash cost of production (CCOP) is the sum of the fixed and variable production costs.

''CCOP = VCOP + FCOP''

where VCOP is the variable cost of production and FCOP is the fixed cost of production.

Gross profit, which should not be confused with gross margin, is then calculated by the following equation,

''Gross profit = Main product revenues - CCOP''

Finally profit can be calculated by subtracting the income taxes that the plant would be subject to depending on the tax code of the county the plant is located in.

''Net profit = gross profit - taxes''

==Pricing Products and Raw Materials==
The revenues and costs of a project are vital to determining its economic feasibility. To calculate these values one needs to multiply the respective product and feed streams by their respective prices. The major difficulty of this process is determining the prices that should be used in this formula. When analyzing a plant, not only do the current prices need to be acknowledged but also the stability of the market to forecast future fluctuations and deviations.
===Pricing Fundamentals===
The pricing of a substance is determined by the fundamental economic principles of supply and demand. A supply curve and demand curve can be graphed and added to determine the market equilibrium price and projected market size. There are many ways a company can combat if the market equilibrium pricing is not suitable for a process. One of these ways is changing the market that the company is selling to. Instead of selling industrial grade product there may be markets for pharmaceutical grade or food grade that would allow for a company to sell their product at higher margins. Another avenue to look into is changing the geographic market being sold to. Rarely is there a global synchronous market, but rather a variation depending on where in the world the product is being sold. It is possible that a company could make more money by dedicating their sales to the Asian market as opposed to the US or vise versa.
===Price Data Sources===
There are many resources when trying to determine the price of a chemical or utility. This are important for looking at current pricing information as well as historical data that can be used for forecasting purposes.
====Internal Company Forecasts====
Large companies will often have the marketing or development departments develop a forecasting database that can be used internally in the company. Forecasts of this magnitude will often have multiple scenarios and projects that are evaluated under the given parameters. Companies may even license these forecasts to other companies for high fees if they desire.

[[File:Capture.JPG]]

Table 1 provides common industry acronyms that are used to indicate certain key words when determining pricing information.

====Trade Journals====
There are also many publications that report pricing data weekly. ''ICIS Chemical Business Americas'' used to publish the prices for hundreds of chemicals but have more recently changed their data to an online database that requires a subscription. This service is very expensive, but necessary for many companies. ''Oil and Gas Journal'' publishes the market prices of many crude oils and other petrochemicals using data from several continents. This journal also provides margin data for many refineries and plants on a monthly basis. ''Chemical Week'' provides the spot and contract prices for 22 chemicals in the US and European markets.

====Consultants====
If trade journals are not adequate for the information needed, some companies will contract consultants to do deep research into the subject. Consultants are excellent resources for providing economic and marketing information but come at a large price. There are several companies that provide this type of service but some of the larger firms include: ''Purvin and Gertz'', ''Cambidge Energy Research Associates'', ''Chemical Markets Associates Inc.'', and ''SRI: The Chemical Economics Handbook''

====Online Brokers and Suppliers====
Often time price data can be supplied by the supplier themselves and using online directories. Restraint should be used when quoting these prices however because they are often spot prices that are much higher than what would be expected from bulk contract supplying.

==Example Case: Estimating Cost of Production==

Use the following information to estimate the manufacturing cost of a nitric acid plant producing 92,000 tonnes/year.

:Fixed Capital Investment: $11,000,000
:Raw Material Cost: $7,950,000/yr
:Waste Treatment Cost: $1,000,000/yr
:Utilities: $356,000/year
:Direct Labor Cost: $300,000/yr
:Fixed Costs: $1,500,000/yr

To find the manufacturing cost, multiply the value of the variable cost by the appropriate cost factor. In this case, we get,

<math>COM_d = (0.180)($11,000,000) + (2.73)($300,000) + (1.23($356,000 + $1,000,000 + $7,950,000) = $14,245,000/yr</math>

<math>($14,245,000/yr)/(92,000 tonne/yr) = $155/tonne</math>

==References==
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-24T05:48:49Z

Jxamplas: /* Example Case: Estimating Cost of Production */

==Variable Cost of Production==
Variable costs of production are dependent primarily on plant output and rate of production. There are many variables to consider when costing a plant.
# Raw materials consumed
# Utilities-steam, electricity, cooling water, fuel, etc.
# Consumables - acids, bases, solvents, catalysts, etc.
# Disposal
# Shipping
The majority of the variable costs for a production plant are the raw materials and utilities costs. Variable costs can be greatly cut through optimization techniques and intelligent plant design [1].
===Raw Materials Cost===
Calculating the annual cost of a raw material is calculated by simply multiplying the feed rate of the process by the appropriate price per volume or mass. These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [5].There are several ways to optimize this cost to ensure that a process is not costing more than it should. First one should assess the actual consumption of a plant to see if it is significantly different from what should be expected based on process stoichiometry and selectivities [1]. Finding may prove that a process is less efficient than it originally claimed.
It is smart to benchmark a new plant design against an existing plant or pilot plant. Raw materials are typically the largest contributor to overall variable costs. For bulk chemicals and petrochemicals, raw materials represent 80-90% of the total cash cost of production (CCOP).
===Utilities Cost===
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [5]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

Utility streams are excellent ways to streamline a process and are often indicative of how efficient of a process the project is. Process methods such as steam generation and pinch analysis can be used to greatly reduce utility costs across a plant. Further analysis of pinch analysis techniques and optimizing heat exchanger networks can be found in plant design texts such as first reference from Gavin Towler. The determination of process utility costs is often more difficult than the determination of raw material costs; however, the utilities are typically between 5-10% of CCOP [1]. The cost of heating a process can be reduced by using process waste streams as fuel which consequently also reduces the need for waste disposal.

===Waste Disposal Costs===
These are defined as the cost of waste treatment to protect the environment [5]. These are materials that cannot be recycled or sold off as by-products. Often times these streams require additives or additional treatment to meet governmental regulations.
Hydrocarbon waste can often be incinerated directly to the atmosphere or used as process fuel to heat other streams in the system. Using the stream as process fuel allows the fuel value of the stream to be recovered into the system. The substituted value can be calculated by multiplying the conventional fuel price by the heat of combustion of the waste stream.

<math>P_{WFV} = P_F * \Delta H_C^o</math>

where
<math>P_{WFV}</math> = waste value of fuel ($/lb or $/kg)

<math>P_F</math> = price of fuel ($/MMBtu or $/GJ)

<math>\Delta H_C^o</math> = heat of combustion (MMBtu/lb or GJ/kg)

Dilute aqueous streams must be sent to wastewater treatment typically prior to purging from the plant. Acidic or basic wastes are neutralized prior to treatment by salting out the acid or base. The cost of wastewater treatment is typically about $6 per 1000 gal but this is only an estimate that doesn't account for regional charges [1].

Solid waste treatment can typically be sent to a landfill at a cost of approximately $50/ton [1].

Hazardous wastes arise from the production of concentrated liquid streams that cannot be incinerated. Hazardous wastes should be avoided if possible, but that is not always feasible for some processes. The cost of hazardous waste disposal is strongly dependent on the location of the plant, the plants proximity to waste disposal plants and the degree of hazard of the waste.

==Fixed Cost of Production==
Fixed costs are those whose amounts are independent of production rates. Much of these costs are personnel salaries, rent, taxes, insurance, and legal payments.
===Labor Costs===
These are the costs attributed to the personnel required to operate the process plant [5].

The number of operators required per shift, <math>N_{OL}</math> can be calculated by

<math>N_{OL}=(6.29+31.7P^2+0.23N_{np})^{0.5}</math>

where <math>P</math> is the number of processing steps involving particulate solids and <math>N_{np}</math> is the number of other processing steps.[5] For each of the <math>N_{OL}</math> operators per 8-hour shift, approximately 4.5 operators must be hired for a plant that runs 24 hours per day, to account for the 3 shifts per day and the 3 weeks of leave typically taken by each operator per year.[5] The salary for a chemical plant operator varies by location, and the estimator should look up the average value for the area.

===Maintenance Costs===
Part of these costs are associated with labor and materials necessary to maintain plant production [5].

===Research and Development===
These are the costs of research done in developing the process and/or products. This includes salaries for researchers as well as funds for research related equipment and supplies [5].

===Land, Rent, Taxes===
===Insurance===
===Interest Payments===
===Corporate Overhead===
===Licensing and Royalties===

==Revenues==
The revenues of a process are the income earned form sales of the main products and the by-products. Revenue can be impacted by market fluctuations and production rates.
===By-Product Revenues===
Besides selling the main product from a process, by-products from separations and reactions can also be valuable in the market. Often it is more difficult to decide which by-products to recover and purify than it is to make decisions on the main product.

By-products made in stoichiometric ratios from reactions must be either sold off or managed through waste disposal. Other by-products are sometimes produced through feed impurities or by nonselective reactions. There are several potential valuable by-products from a process:
# Materials produced in stoichiometric quantities by the reactions that create the main product. If they are not recovered then the waste disposal expenses will be large.
# Components that are produced in high yield by side reactions.
# Components formed in high yield from feed impurities. Many sulfurs are produced as a by-product of fuels manufacture.
# Components that are produced in low yield but have high value. An example includes acetophenone which is recovered as a by-product of phenol manufacture.
# Degraded consumables (e.g. solvents, etc.) that have reuse value.

A rule of thumb that can be used for preliminary screening of by-products for large plants is that for by-product recovery to be economically feasible the net benefit must be greater than $200,000 a year. A net benefit can be calculated by adding the possible resale value of the by-product and the avoided waste disposal cost [1].

===Margin===
The gross margin of a process is defined as the sum of product and by-product revenues minus the raw material cost.

Gross margin = Revenues - Raw materials costs

Because raw materials are most often the most expensive variable cost of a process, the gross margin is a good gauge as to what the total profitability of a process will be. Raw materials and product pricing are often subject to high degrees of variability which can be difficult to forecast. The size of margins are highly versatile depending on the
industry. For many petrochemical industries the margin may be only 10%; however, for industries such as food additives and pharmaceuticals the margins are generally much higher [1].

===Profits===
There are several standards for calculating company profits. The cash cost of production (CCOP) is the sum of the fixed and variable production costs.

''CCOP = VCOP + FCOP''

where VCOP is the variable cost of production and FCOP is the fixed cost of production.

Gross profit, which should not be confused with gross margin, is then calculated by the following equation,

''Gross profit = Main product revenues - CCOP''

Finally profit can be calculated by subtracting the income taxes that the plant would be subject to depending on the tax code of the county the plant is located in.

''Net profit = gross profit - taxes''

==Pricing Products and Raw Materials==
The revenues and costs of a project are vital to determining its economic feasibility. To calculate these values one needs to multiply the respective product and feed streams by their respective prices. The major difficulty of this process is determining the prices that should be used in this formula. When analyzing a plant, not only do the current prices need to be acknowledged but also the stability of the market to forecast future fluctuations and deviations.
===Pricing Fundamentals===
The pricing of a substance is determined by the fundamental economic principles of supply and demand. A supply curve and demand curve can be graphed and added to determine the market equilibrium price and projected market size. There are many ways a company can combat if the market equilibrium pricing is not suitable for a process. One of these ways is changing the market that the company is selling to. Instead of selling industrial grade product there may be markets for pharmaceutical grade or food grade that would allow for a company to sell their product at higher margins. Another avenue to look into is changing the geographic market being sold to. Rarely is there a global synchronous market, but rather a variation depending on where in the world the product is being sold. It is possible that a company could make more money by dedicating their sales to the Asian market as opposed to the US or vise versa.
===Price Data Sources===
There are many resources when trying to determine the price of a chemical or utility. This are important for looking at current pricing information as well as historical data that can be used for forecasting purposes.
====Internal Company Forecasts====
Large companies will often have the marketing or development departments develop a forecasting database that can be used internally in the company. Forecasts of this magnitude will often have multiple scenarios and projects that are evaluated under the given parameters. Companies may even license these forecasts to other companies for high fees if they desire.

[[File:Capture.JPG]]

Table 1 provides common industry acronyms that are used to indicate certain key words when determining pricing information.

====Trade Journals====
There are also many publications that report pricing data weekly. ''ICIS Chemical Business Americas'' used to publish the prices for hundreds of chemicals but have more recently changed their data to an online database that requires a subscription. This service is very expensive, but necessary for many companies. ''Oil and Gas Journal'' publishes the market prices of many crude oils and other petrochemicals using data from several continents. This journal also provides margin data for many refineries and plants on a monthly basis. ''Chemical Week'' provides the spot and contract prices for 22 chemicals in the US and European markets.

====Consultants====
If trade journals are not adequate for the information needed, some companies will contract consultants to do deep research into the subject. Consultants are excellent resources for providing economic and marketing information but come at a large price. There are several companies that provide this type of service but some of the larger firms include: ''Purvin and Gertz'', ''Cambidge Energy Research Associates'', ''Chemical Markets Associates Inc.'', and ''SRI: The Chemical Economics Handbook''

====Online Brokers and Suppliers====
Often time price data can be supplied by the supplier themselves and using online directories. Restraint should be used when quoting these prices however because they are often spot prices that are much higher than what would be expected from bulk contract supplying.

==Example Case: Estimating Cost of Production==

Use the following information to estimate the manufacturing cost of a nitric acid plant producing 92,000 tonnes/year.

:Fixed Capital Investment: $11,000,000
:Raw Material Cost: $7,950,000/yr
:Waste Treatment Cost: $1,000,000/yr
:Utilities: $356,000/year
:Direct Labor Cost: $300,000/yr
:Fixed Costs: $1,500,000/yr

To find the manufacturing cost, multiply the value of the variable cost by the appropriate cost factor. In this case, we get,

<math>COM_d = (0.180)($11,000,000) + (2.73)($300,000) + (1.23($356,000 + $1,000,000 + $7,950,000) = $14,245,000/yr</math>

<math>($14,245,000/yr)/(92,000 tonne/yr) = $155/tonne</math>

==References==
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-24T05:47:18Z

Jxamplas:

==Variable Cost of Production==
Variable costs of production are dependent primarily on plant output and rate of production. There are many variables to consider when costing a plant.
# Raw materials consumed
# Utilities-steam, electricity, cooling water, fuel, etc.
# Consumables - acids, bases, solvents, catalysts, etc.
# Disposal
# Shipping
The majority of the variable costs for a production plant are the raw materials and utilities costs. Variable costs can be greatly cut through optimization techniques and intelligent plant design [1].
===Raw Materials Cost===
Calculating the annual cost of a raw material is calculated by simply multiplying the feed rate of the process by the appropriate price per volume or mass. These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [5].There are several ways to optimize this cost to ensure that a process is not costing more than it should. First one should assess the actual consumption of a plant to see if it is significantly different from what should be expected based on process stoichiometry and selectivities [1]. Finding may prove that a process is less efficient than it originally claimed.
It is smart to benchmark a new plant design against an existing plant or pilot plant. Raw materials are typically the largest contributor to overall variable costs. For bulk chemicals and petrochemicals, raw materials represent 80-90% of the total cash cost of production (CCOP).
===Utilities Cost===
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [5]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

Utility streams are excellent ways to streamline a process and are often indicative of how efficient of a process the project is. Process methods such as steam generation and pinch analysis can be used to greatly reduce utility costs across a plant. Further analysis of pinch analysis techniques and optimizing heat exchanger networks can be found in plant design texts such as first reference from Gavin Towler. The determination of process utility costs is often more difficult than the determination of raw material costs; however, the utilities are typically between 5-10% of CCOP [1]. The cost of heating a process can be reduced by using process waste streams as fuel which consequently also reduces the need for waste disposal.

===Waste Disposal Costs===
These are defined as the cost of waste treatment to protect the environment [5]. These are materials that cannot be recycled or sold off as by-products. Often times these streams require additives or additional treatment to meet governmental regulations.
Hydrocarbon waste can often be incinerated directly to the atmosphere or used as process fuel to heat other streams in the system. Using the stream as process fuel allows the fuel value of the stream to be recovered into the system. The substituted value can be calculated by multiplying the conventional fuel price by the heat of combustion of the waste stream.

<math>P_{WFV} = P_F * \Delta H_C^o</math>

where
<math>P_{WFV}</math> = waste value of fuel ($/lb or $/kg)

<math>P_F</math> = price of fuel ($/MMBtu or $/GJ)

<math>\Delta H_C^o</math> = heat of combustion (MMBtu/lb or GJ/kg)

Dilute aqueous streams must be sent to wastewater treatment typically prior to purging from the plant. Acidic or basic wastes are neutralized prior to treatment by salting out the acid or base. The cost of wastewater treatment is typically about $6 per 1000 gal but this is only an estimate that doesn't account for regional charges [1].

Solid waste treatment can typically be sent to a landfill at a cost of approximately $50/ton [1].

Hazardous wastes arise from the production of concentrated liquid streams that cannot be incinerated. Hazardous wastes should be avoided if possible, but that is not always feasible for some processes. The cost of hazardous waste disposal is strongly dependent on the location of the plant, the plants proximity to waste disposal plants and the degree of hazard of the waste.

==Fixed Cost of Production==
Fixed costs are those whose amounts are independent of production rates. Much of these costs are personnel salaries, rent, taxes, insurance, and legal payments.
===Labor Costs===
These are the costs attributed to the personnel required to operate the process plant [5].

The number of operators required per shift, <math>N_{OL}</math> can be calculated by

<math>N_{OL}=(6.29+31.7P^2+0.23N_{np})^{0.5}</math>

where <math>P</math> is the number of processing steps involving particulate solids and <math>N_{np}</math> is the number of other processing steps.[5] For each of the <math>N_{OL}</math> operators per 8-hour shift, approximately 4.5 operators must be hired for a plant that runs 24 hours per day, to account for the 3 shifts per day and the 3 weeks of leave typically taken by each operator per year.[5] The salary for a chemical plant operator varies by location, and the estimator should look up the average value for the area.

===Maintenance Costs===
Part of these costs are associated with labor and materials necessary to maintain plant production [5].

===Research and Development===
These are the costs of research done in developing the process and/or products. This includes salaries for researchers as well as funds for research related equipment and supplies [5].

===Land, Rent, Taxes===
===Insurance===
===Interest Payments===
===Corporate Overhead===
===Licensing and Royalties===

==Revenues==
The revenues of a process are the income earned form sales of the main products and the by-products. Revenue can be impacted by market fluctuations and production rates.
===By-Product Revenues===
Besides selling the main product from a process, by-products from separations and reactions can also be valuable in the market. Often it is more difficult to decide which by-products to recover and purify than it is to make decisions on the main product.

By-products made in stoichiometric ratios from reactions must be either sold off or managed through waste disposal. Other by-products are sometimes produced through feed impurities or by nonselective reactions. There are several potential valuable by-products from a process:
# Materials produced in stoichiometric quantities by the reactions that create the main product. If they are not recovered then the waste disposal expenses will be large.
# Components that are produced in high yield by side reactions.
# Components formed in high yield from feed impurities. Many sulfurs are produced as a by-product of fuels manufacture.
# Components that are produced in low yield but have high value. An example includes acetophenone which is recovered as a by-product of phenol manufacture.
# Degraded consumables (e.g. solvents, etc.) that have reuse value.

A rule of thumb that can be used for preliminary screening of by-products for large plants is that for by-product recovery to be economically feasible the net benefit must be greater than $200,000 a year. A net benefit can be calculated by adding the possible resale value of the by-product and the avoided waste disposal cost [1].

===Margin===
The gross margin of a process is defined as the sum of product and by-product revenues minus the raw material cost.

Gross margin = Revenues - Raw materials costs

Because raw materials are most often the most expensive variable cost of a process, the gross margin is a good gauge as to what the total profitability of a process will be. Raw materials and product pricing are often subject to high degrees of variability which can be difficult to forecast. The size of margins are highly versatile depending on the
industry. For many petrochemical industries the margin may be only 10%; however, for industries such as food additives and pharmaceuticals the margins are generally much higher [1].

===Profits===
There are several standards for calculating company profits. The cash cost of production (CCOP) is the sum of the fixed and variable production costs.

''CCOP = VCOP + FCOP''

where VCOP is the variable cost of production and FCOP is the fixed cost of production.

Gross profit, which should not be confused with gross margin, is then calculated by the following equation,

''Gross profit = Main product revenues - CCOP''

Finally profit can be calculated by subtracting the income taxes that the plant would be subject to depending on the tax code of the county the plant is located in.

''Net profit = gross profit - taxes''

==Pricing Products and Raw Materials==
The revenues and costs of a project are vital to determining its economic feasibility. To calculate these values one needs to multiply the respective product and feed streams by their respective prices. The major difficulty of this process is determining the prices that should be used in this formula. When analyzing a plant, not only do the current prices need to be acknowledged but also the stability of the market to forecast future fluctuations and deviations.
===Pricing Fundamentals===
The pricing of a substance is determined by the fundamental economic principles of supply and demand. A supply curve and demand curve can be graphed and added to determine the market equilibrium price and projected market size. There are many ways a company can combat if the market equilibrium pricing is not suitable for a process. One of these ways is changing the market that the company is selling to. Instead of selling industrial grade product there may be markets for pharmaceutical grade or food grade that would allow for a company to sell their product at higher margins. Another avenue to look into is changing the geographic market being sold to. Rarely is there a global synchronous market, but rather a variation depending on where in the world the product is being sold. It is possible that a company could make more money by dedicating their sales to the Asian market as opposed to the US or vise versa.
===Price Data Sources===
There are many resources when trying to determine the price of a chemical or utility. This are important for looking at current pricing information as well as historical data that can be used for forecasting purposes.
====Internal Company Forecasts====
Large companies will often have the marketing or development departments develop a forecasting database that can be used internally in the company. Forecasts of this magnitude will often have multiple scenarios and projects that are evaluated under the given parameters. Companies may even license these forecasts to other companies for high fees if they desire.

[[File:Capture.JPG]]

Table 1 provides common industry acronyms that are used to indicate certain key words when determining pricing information.

====Trade Journals====
There are also many publications that report pricing data weekly. ''ICIS Chemical Business Americas'' used to publish the prices for hundreds of chemicals but have more recently changed their data to an online database that requires a subscription. This service is very expensive, but necessary for many companies. ''Oil and Gas Journal'' publishes the market prices of many crude oils and other petrochemicals using data from several continents. This journal also provides margin data for many refineries and plants on a monthly basis. ''Chemical Week'' provides the spot and contract prices for 22 chemicals in the US and European markets.

====Consultants====
If trade journals are not adequate for the information needed, some companies will contract consultants to do deep research into the subject. Consultants are excellent resources for providing economic and marketing information but come at a large price. There are several companies that provide this type of service but some of the larger firms include: ''Purvin and Gertz'', ''Cambidge Energy Research Associates'', ''Chemical Markets Associates Inc.'', and ''SRI: The Chemical Economics Handbook''

====Online Brokers and Suppliers====
Often time price data can be supplied by the supplier themselves and using online directories. Restraint should be used when quoting these prices however because they are often spot prices that are much higher than what would be expected from bulk contract supplying.

==Example Case: Estimating Cost of Production==

Use the following information to estimate the manufacturing cost of a nitric acid plant producing 92,000 tonnes/year.

:Fixed Capital Investment: $11,000,000
:Raw Material Cost: $7,950,000/yr
:Waste Treatment Cost: $1,000,000/yr
:Utilities: $356,000/year
:Direct Labor Cost: $300,000/yr
:Fixed Costs: $1,500,000/yr

To find the manufacturing cost, multiply the value of the variable cost by the appropriate cost factor. In this case, we get,

<math>COM_d = (0.180)($11,000,000) + (2.73)($300,000) + (1.23($356,000 + $1,000,000 + $7,950,000) = $14,245,000/yr</math>

<math> ($14,245,000/yr)/(92,000 tonne/yr) = $155/tonne</math>

==References==
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-22T22:36:42Z

Jxamplas: /* Price data Sources */

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [6].

====Utilities Cost====
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [6]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

====Consumables Cost====
====Waste Disposal Costs====
These are defined as the cost of waste treatment to protect the environment [6].

==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
These are the costs attributed to the personnel required to operate the process plant [6].

====Maintenance Costs====
Part of these costs are associated with labor and materials necessary to maintain plant production [6].

====Research and Development====
These are the costs of research done in developing the process and/or products. This includes salaries for researchers as well as funds for research related equipment and supplies [6].

====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====

==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====

===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-22T22:36:07Z

Jxamplas: /* Fixed Cost of Production */

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [6].

====Utilities Cost====
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [6]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

====Consumables Cost====
====Waste Disposal Costs====
These are defined as the cost of waste treatment to protect the environment [6].

==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
These are the costs attributed to the personnel required to operate the process plant [6].

====Maintenance Costs====
Part of these costs are associated with labor and materials necessary to maintain plant production [6].

====Research and Development====
These are the costs of research done in developing the process and/or products. This includes salaries for researchers as well as funds for research related equipment and supplies [6].

====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====

==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====
===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-22T22:33:34Z

Jxamplas: /* Maintenance Costs */

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [6].

====Utilities Cost====
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [6]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

====Consumables Cost====
====Waste Disposal Costs====
These are defined as the cost of waste treatment to protect the environment [6].

==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
These are the costs attributed to the personnel required to operate the process plant [6].

====Maintenance Costs====
Part of these costs are associated with labor and materials necessary to maintain plant production [6].

====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====
==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====
===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-22T22:32:49Z

Jxamplas: /* Labor Costs */

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [6].

====Utilities Cost====
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [6]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

====Consumables Cost====
====Waste Disposal Costs====
These are defined as the cost of waste treatment to protect the environment [6].

==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
These are the costs attributed to the personnel required to operate the process plant [6].

====Maintenance Costs====
====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====
==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====
===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-22T22:28:02Z

Jxamplas: /* Utilities Cost */

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [6].

====Utilities Cost====
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD [6]. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

====Consumables Cost====
====Waste Disposal Costs====
These are defined as the cost of waste treatment to protect the environment [6].

==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
====Maintenance Costs====
====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====
==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====
===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-22T22:27:51Z

Jxamplas: /* Utilities Cost */

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [6].

====Utilities Cost====
These are the costs of the various utilities streams required by the process. The flowrates for the utilities streams are located on the PFD. This includes:
*Fuel gas, oil, or coal
*Electric power
*Steam
*Cooling water
*Process water
*Boiler feed water
*Air
*Inert gas
*Refrigeration

====Consumables Cost====
====Waste Disposal Costs====
These are defined as the cost of waste treatment to protect the environment [6].

==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
====Maintenance Costs====
====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====
==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====
===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-22T22:24:03Z

Jxamplas: /* Raw Materials Cost */

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
These are the costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [6].

====Utilities Cost====
====Consumables Cost====
====Waste Disposal Costs====
These are defined as the cost of waste treatment to protect the environment [6].

==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
====Maintenance Costs====
====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====
==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====
===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-22T22:23:39Z

Jxamplas: /* Waste Disposal Costs */

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
Costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [6].

====Utilities Cost====
====Consumables Cost====
====Waste Disposal Costs====
These are defined as the cost of waste treatment to protect the environment [6].

==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
====Maintenance Costs====
====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====
==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====
===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-22T22:22:45Z

Jxamplas: /* Raw Materials Cost */

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
Costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD [6].

====Utilities Cost====
====Consumables Cost====
====Waste Disposal Costs====
==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
====Maintenance Costs====
====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====
==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====
===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Separation processes

2014-02-22T22:22:19Z

Jxamplas: /* Example Case: Ideal Distillation */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Distillation Applications===

Distillation is a process that can be implemented in various scales. There is both laboratory scaled distillation as well as very large industrial distillation. Other applications for distillation include food/alcohol processing and herb distillation for the perfume and medical industries. Typically laboratory scaled distillation occurs in batches whereas industrial distillation (e.g. fractional distillation of crude oil) occurs continuous with a constant distillate and bottom effluent streams.

Some applications of distillation are concerned the top stream only, some the bottom stream only and others both streams can be used for future products. In alcohol distillation for example, the water that is separated from the ethanol/water binary solution is discarded as waste water. In fractional distillation of crude oils, the heavy hydrocarbons at the bottom of the column are collected and sold along with the light hydrocarbons that appear in higher side draws [1].

===Example Case: Ideal Distillation===

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the heaviest product, heptane, and then separates pentane from hexane in the second column. This example will consider the direct sequence. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. Also, it is a good idea to look up relative volatilites, to further verify near-ideality of the mixture, but also to obtain the information necessary for the Underwood method, which we will employ to obtain a solution. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_{II}(nC5) = 2 mol/s</math>

<math>\mu_I(nC6) + \mu_{II}(nC6) + \mu_{III}(nC6) = 3 mol/s</math>

<math>\mu_{II}(nC7) + \mu_{III}(nC7) = 5 mol/s</math>

<math>\mu_I(nC5) = 99\mu_I(nC6)</math>

<math>\mu_{II}(nC5) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{II}(nC7) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{III}(nC7) = 99\mu_{III}(nC7)</math>

where <math>\mu</math> represents the molar flow, and the subscript represents the product stream.

Solving this system of equations:

<math>\mu_I(nC5) = 1.985\ mol/s</math>

<math>\mu_{II}(nC5) = 0.015\ mol/s</math>

<math>\mu_I(nC6) = 0.020\ mol/s</math>

<math>\mu_{II}(nC6) = 2.930\ mol/s</math>

<math>\mu_{III}(nC6) = 0.050\ mol/s</math>

<math>\mu_{II}(nC7) = 0.015\ mol/s</math>

<math>\mu_{III}(nC7) = 4.985\ mol/s</math>

At this point we have enough information to use Underwood's method to estimate the minimum vapor flows in the column. The following three equations are used in Underwood's method:

<math>\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}f_i = (1-q)F</math>

<math>(R_{min}+1)D = \sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}d_i = V_{min}</math>

<math>\bar R_{min}B = -\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}b_i = \bar V_{min}</math>

where <math>\alpha_{ik}</math> is the relative volatility of species <math>i</math> to species <math>k</math>, <math>f_i</math> the molar flow of species <math>i</math> in the feed, <math>q</math> the fraction of the feed that joins the liquid stream at the feed tray, <math>F</math> the total molar flow of the feed, <math>D</math> the molar flow of the distillate, <math>R_{min}</math> the minimum reflux ratio <math>(=L_{min}/D)</math>, <math>d_i</math> the molar flow of species <math>i</math> in the distillate, <math>V_{min}</math> the minimum vapor flow possible in the top section of the column to accomplish the desired separation, <math>\bar R_{min}</math> the minimum reboil ratio <math>(=\bar V_{min}/B)</math>, <math>b_i</math> the molar flow of species <math>i</math> in the bottoms product, and <math>\bar V_{min}</math> the minimum vapor flow in the bottom section of the column. The final variable, <math>\phi</math>, will be solved for using the first Underwood equation, and it's value will be decided based on the relative volatilities of the key components in the column.

So, after solving the first Underwood equation, we get two values for <math>\phi</math>, 3.806 and 1.462. Because 3.806 is between the relative volatilities of the key components, we will substitute that value for <math>\phi</math> into the second Underwood equation. Doing so for both columns gives <math>V_{min} = 6.4\ mol/s</math> for the first column and <math>V_{min} = 8.9\ mol/s</math> for the second column, for a total minimum vapor flow of 15.3 mol/s. The process would then be repeated for the indirect sequence, and the decision for which process to use would be justified by the process with the overall minimum vapor flow [3].

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

An industrial example is lean oil absorption, which is used to separate nitrogen and other impurities from natural gas. A lean oil is contacted with low quality natural gas, and the methane is selectively absorbed by the lean oil, leaving the impurities behind. The methane is subsequently regenerated from the rich oil as high quality natural gas [7].

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [8].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==Conclusion==

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.
# Lean Oil Absorption. PetroGas Systems Web site. Available at: http://petrogassystems.com/technology/natural-gas-processing-and-dew-point-control/lean-oil-absorption. Accessed February 19, 2014.

Separation processes

2014-02-22T22:22:00Z

Jxamplas: /* Example Case: Ideal Distillation */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Distillation Applications===

Distillation is a process that can be implemented in various scales. There is both laboratory scaled distillation as well as very large industrial distillation. Other applications for distillation include food/alcohol processing and herb distillation for the perfume and medical industries. Typically laboratory scaled distillation occurs in batches whereas industrial distillation (e.g. fractional distillation of crude oil) occurs continuous with a constant distillate and bottom effluent streams.

Some applications of distillation are concerned the top stream only, some the bottom stream only and others both streams can be used for future products. In alcohol distillation for example, the water that is separated from the ethanol/water binary solution is discarded as waste water. In fractional distillation of crude oils, the heavy hydrocarbons at the bottom of the column are collected and sold along with the light hydrocarbons that appear in higher side draws [1].

===Example Case: Ideal Distillation===

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the heaviest product, heptane, and then separates pentane from hexane in the second column. This example will consider the direct sequence. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. Also, it is a good idea to look up relative volatilites, to further verify near-ideality of the mixture, but also to obtain the information necessary for the Underwood method, which we will employ to obtain a solution. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_{II}(nC5) = 2 mol/s</math>

<math>\mu_I(nC6) + \mu_{II}(nC6) + \mu_{III}(nC6) = 3 mol/s</math>

<math>\mu_{II}(nC7) + \mu_{III}(nC7) = 5 mol/s</math>

<math>\mu_I(nC5) = 99\mu_I(nC6)</math>

<math>\mu_{II}(nC5) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{II}(nC7) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{III}(nC7) = 99\mu_{III}(nC7)</math>

where <math>\mu</math> represents the molar flow, and the subscript represents the product stream.

Solving this system of equations:

<math>\mu_I(nC5) = 1.985\ mol/s</math>

<math>\mu_{II}(nC5) = 0.015\ mol/s</math>

<math>\mu_I(nC6) = 0.020\ mol/s</math>

<math>\mu_{II}(nC6) = 2.930\ mol/s</math>

<math>\mu_{III}(nC6) = 0.050\ mol/s</math>

<math>\mu_{II}(nC7) = 0.015\ mol/s</math>

<math>\mu_{III}(nC7) = 4.985\ mol/s</math>

At this point we have enough information to use Underwood's method to estimate the minimum vapor flows in the column. The following three equations are used in Underwood's method:

<math>\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}f_i = (1-q)F</math>

<math>(R_{min}+1)D = \sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}d_i = V_{min}</math>

<math>\bar R_{min}B = -\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}b_i = \bar V_{min}</math>

where <math>\alpha_{ik}</math> is the relative volatility of species <math>i</math> to species <math>k</math>, <math>f_i</math> the molar flow of species <math>i</math> in the feed, <math>q</math> the fraction of the feed that joins the liquid stream at the feed tray, <math>F</math> the total molar flow of the feed, <math>D</math> the molar flow of the distillate, <math>R_{min}</math> the minimum reflux ratio <math>(=L_{min}/D)</math>, <math>d_i</math> the molar flow of species <math>i</math> in the distillate, <math>V_{min}</math> the minimum vapor flow possible in the top section of the column to accomplish the desired separation, <math>\bar R_{min}</math> the minimum reboil ratio <math>(=\bar V_{min}/B)</math>, <math>b_i</math> the molar flow of species <math>i</math> in the bottoms product, and <math>\bar V_{min}</math> the minimum vapor flow in the bottom section of the column. The final variable, <math>\phi</math>, will be solved for using the first Underwood equation, and it's value will be decided based on the relative volatilities of the key components in the column.

So, after solving the first Underwood equation, we get two values for <math>\phi</math>, 3.806 and 1.462. Because 3.806 is between the relative volatilities of the key components, we will substitute that value for <math>\phi</math> into the second Underwood equation. Doing so for both columns gives <math>V_{min} = 6.4\ mol/s</math> for the first column and <math>V_{min} = 8.9\ mol/s</math> for the second column, for a total minimum vapor flow of 15.3 mol/s. The process would then be repeated for the indirect sequence, and the decision for which process to use would be justified by the process with the overall minimum vapor flow. [3]

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

An industrial example is lean oil absorption, which is used to separate nitrogen and other impurities from natural gas. A lean oil is contacted with low quality natural gas, and the methane is selectively absorbed by the lean oil, leaving the impurities behind. The methane is subsequently regenerated from the rich oil as high quality natural gas [7].

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [8].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==Conclusion==

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.
# Lean Oil Absorption. PetroGas Systems Web site. Available at: http://petrogassystems.com/technology/natural-gas-processing-and-dew-point-control/lean-oil-absorption. Accessed February 19, 2014.

Estimation of production cost and revenue

2014-02-22T22:21:03Z

Jxamplas: /* References */

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
Costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD.

====Utilities Cost====
====Consumables Cost====
====Waste Disposal Costs====
==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
====Maintenance Costs====
====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====
==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====
===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Estimation of production cost and revenue

2014-02-22T22:20:50Z

Jxamplas:

==Variable Cost of Production==
===Estimating Variable Production Costs===
====Raw Materials Cost====
Costs of chemical feed stocks required by the process. Feed stocks flow rates are obtained from PFD.

====Utilities Cost====
====Consumables Cost====
====Waste Disposal Costs====
==Fixed Cost of Production==
===Estimating Fixed Production Costs===
====Labor Costs====
====Maintenance Costs====
====Land, Rent, Taxes====
====Insurance====
====Interest Payments====
====Corporate Overhead====
====Licensing and Royalties====
==Revenues==
===By-Product Revenues===
===Margin===
===Profits===
==Pricing Products and Raw Materials==
===Pricing Fundamentals===
===Price data Sources===
====Internal Company Forecasts====
====Trade Journals====
====Consultants====
====Online Brokers and Suppliers====
====Reference Books====
===Price Forecasting===
==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.
# Lean Oil Absorption. PetroGas Systems Web site. Available at: http://petrogassystems.com/technology/natural-gas-processing-and-dew-point-control/lean-oil-absorption. Accessed February 19, 2014.

Estimation of production cost and revenue

2014-02-22T22:19:47Z

Jxamplas: /* Raw Materials Cost */

Separation processes

2014-02-20T19:30:45Z

Jxamplas: /* Example Case: Ideal Distillation */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Example Case: Ideal Distillation===

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the heaviest product, heptane, and then separates pentane from hexane in the second column. This example will consider the direct sequence. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. Also, it is a good idea to look up relative volatilites, to further verify near-ideality of the mixture, but also to obtain the information necessary for the Underwood method, which we will employ to obtain a solution. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_{II}(nC5) = 2 mol/s</math>

<math>\mu_I(nC6) + \mu_{II}(nC6) + \mu_{III}(nC6) = 3 mol/s</math>

<math>\mu_{II}(nC7) + \mu_{III}(nC7) = 5 mol/s</math>

<math>\mu_I(nC5) = 99\mu_I(nC6)</math>

<math>\mu_{II}(nC5) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{II}(nC7) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{III}(nC7) = 99\mu_{III}(nC7)</math>

where <math>\mu</math> represents the molar flow, and the subscript represents the product stream.

Solving this system of equations:

<math>\mu_I(nC5) = 1.985\ mol/s</math>

<math>\mu_{II}(nC5) = 0.015\ mol/s</math>

<math>\mu_I(nC6) = 0.020\ mol/s</math>

<math>\mu_{II}(nC6) = 2.930\ mol/s</math>

<math>\mu_{III}(nC6) = 0.050\ mol/s</math>

<math>\mu_{II}(nC7) = 0.015\ mol/s</math>

<math>\mu_{III}(nC7) = 4.985\ mol/s</math>

At this point we have enough information to use Underwood's method to estimate the minimum vapor flows in the column. The following three equations are used in Underwood's method:

<math>\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}f_i = (1-q)F</math>

<math>(R_{min}+1)D = \sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}d_i = V_{min}</math>

<math>\bar R_{min}B = -\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}b_i = \bar V_{min}</math>

where <math>\alpha_{ik}</math> is the relative volatility of species <math>i</math> to species <math>k</math>, <math>f_i</math> the molar flow of species <math>i</math> in the feed, <math>q</math> the fraction of the feed that joins the liquid stream at the feed tray, <math>F</math> the total molar flow of the feed, <math>D</math> the molar flow of the distillate, <math>R_{min}</math> the minimum reflux ratio <math>(=L_{min}/D)</math>, <math>d_i</math> the molar flow of species <math>i</math> in the distillate, <math>V_{min}</math> the minimum vapor flow possible in the top section of the column to accomplish the desired separation, <math>\bar R_{min}</math> the minimum reboil ratio <math>(=\bar V_{min}/B)</math>, <math>b_i</math> the molar flow of species <math>i</math> in the bottoms product, and <math>\bar V_{min}</math> the minimum vapor flow in the bottom section of the column. The final variable, <math>\phi</math>, will be solved for using the first Underwood equation, and it's value will be decided based on the relative volatilities of the key components in the column.

So, after solving the first Underwood equation, we get two values for <math>\phi</math>, 3.806 and 1.462. Because 3.806 is between the relative volatilities of the key components, we will substitute that value for <math>\phi</math> into the second Underwood equation. Doing so for both columns gives <math>V_{min} = 6.4\ mol/s</math> for the first column and <math>V_{min} = 8.9\ mol/s</math> for the second column, for a total minimum vapor flow of 15.3 mol/s. The process would then be repeated for the indirect sequence, and the decision for which process to use would be justified by the process with the overall minimum vapor flow.

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

An industrial example is lean oil absorption, which is used to separate nitrogen and other impurities from natural gas. A lean oil is contacted with low quality natural gas, and the methane is selectively absorbed by the lean oil, leaving the impurities behind. The methane is subsequently regenerated from the rich oil as high quality natural gas [7].

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [8].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==Conclusion==

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.
# Lean Oil Absorption. PetroGas Systems Web site. Available at: http://petrogassystems.com/technology/natural-gas-processing-and-dew-point-control/lean-oil-absorption. Accessed February 19, 2014.

Separation processes

2014-02-20T19:30:08Z

Jxamplas: /* Example Case: Ideal Distillation */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Example Case: Ideal Distillation===

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the heaviest product, heptane, and then separates pentane from hexane in the second column. This example will consider the direct sequence. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. Also, it is a good idea to look up relative volatilites, to further verify near-ideality of the mixture, but also to obtain the information necessary for the Underwood method, which we will employ to obtain a solution. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_{II}(nC5) = 2 mol/s</math>

<math>\mu_I(nC6) + \mu_{II}(nC6) + \mu_{III}(nC6) = 3 mol/s</math>

<math>\mu_{II}(nC7) + \mu_{III}(nC7) = 5 mol/s</math>

<math>\mu_I(nC5) = 99\mu_I(nC6)</math>

<math>\mu_{II}(nC5) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{II}(nC7) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{III}(nC7) = 99\mu_{III}(nC7)</math>

where <math>\mu</math> represents the molar flow, and the subscript represents the product stream.

Solving this system of equations:

<math>\mu_I(nC5) = 1.985\ mol/s</math>

<math>\mu_{II}(nC5) = 0.015\ mol/s</math>

<math>\mu_I(nC6) = 0.020\ mol/s</math>

<math>\mu_{II}(nC6) = 2.930\ mol/s</math>

<math>\mu_{III}(nC6) = 0.050\ mol/s</math>

<math>\mu_{II}(nC7) = 0.015\ mol/s</math>

<math>\mu_{III}(nC7) = 4.985\ mol/s</math>

At this point we have enough information to use Underwood's method to estimate the minimum vapor flows in the column. The following three equations are used in Underwood's method:

<math>\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}f_i = (1-q)F</math>

<math>(R_{min}+1)D = \sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}d_i = V_{min}</math>

<math>\bar R_{min}B = -\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}b_i = \bar V_{min}</math>

where <math>\alpha_{ik}</math> is the relative volatility of species <math>i</math> to species <math>k</math>, <math>f_i</math> the molar flow of species <math>i</math> in the feed, <math>q</math> the fraction of the feed that joins the liquid stream at the feed tray, <math>F</math> the total molar flow of the feed, <math>D</math> the molar flow of the distillate, <math>R_{min}</math> the minimum reflux ratio <math>(=L_{min}/D)</math>, <math>d_i</math> the molar flow of species <math>i</math> in the distillate, <math>V_{min}</math> the minimum vapor flow possible in the top section of the column to accomplish the desired separation, <math>\bar R_{min}</math> the minimum reboil ratio <math>(=\bar V_{min}/B)</math>, <math>b_i</math> the molar flow of species <math>i</math> in the bottoms product, and <math>\bar V_{min}</math> the minimum vapor flow in the bottom section of the column. The final variable, <math>\phi</math>, will be solved for using the first Underwood equation, and it's value will be decided based on the relative volatilities of the key components in the column.

So, after solving the first Underwood equation, we get two values for <math>\phi</math>, 3.806 and 1.462. Because 3.806 is between the relative volatilities of the key components, we will substitute that value for <math>\phi</math> into the second Underwood equation. Doing so for both columns gives <math>V_{min} = 6.4\ mol/s</math> for the first and <math>V_{min} = 8.9\ mol/s</math> for the second, for a total minimum vapor flow of 15.3 mol/s. The process would then be repeated for the indirect sequence, and the decision for which process to use would be justified by the process with the overall minimum vapor flow.

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

An industrial example is lean oil absorption, which is used to separate nitrogen and other impurities from natural gas. A lean oil is contacted with low quality natural gas, and the methane is selectively absorbed by the lean oil, leaving the impurities behind. The methane is subsequently regenerated from the rich oil as high quality natural gas [7].

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [8].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==Conclusion==

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.
# Lean Oil Absorption. PetroGas Systems Web site. Available at: http://petrogassystems.com/technology/natural-gas-processing-and-dew-point-control/lean-oil-absorption. Accessed February 19, 2014.

Separation processes

2014-02-20T04:52:33Z

Jxamplas: /* Example Case: Ideal Distillation */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Example Case: Ideal Distillation===

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the haeviest product, heptane, and then separates pentane from hexane in the second column. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. Also, it is a good idea to look up relative volatilites, to further verify near-ideality of the mixture, but also to obtain the information necessary for the Underwood method, which we will employ to obtain a solution. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_{II}(nC5) = 2 mol/s</math>

<math>\mu_I(nC6) + \mu_{II}(nC6) + \mu_{III}(nC6) = 3 mol/s</math>

<math>\mu_{II}(nC7) + \mu_{III}(nC7) = 5 mol/s</math>

<math>\mu_I(nC5) = 99\mu_I(nC6)</math>

<math>\mu_{II}(nC5) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{II}(nC7) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{III}(nC7) = 99\mu_{III}(nC7)</math>

where <math>\mu</math> represents the molar flow, and the subscript represents the product stream.

Solving this system of equations

At this point we have enough information to use Underwood's method to estimate the minimum vapor flows in the column. The following three equations are used in Underwood's method:

<math>\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}f_i = (1-q)F</math>

<math>(R_{min}+1)D = \sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}d_i = V_{min}</math>

<math>\bar R_{min}B = -\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}b_i = \bar V_{min}</math>

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [7].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.

Separation processes

2014-02-20T04:47:12Z

Jxamplas: /* Example Case: Ideal Distillation */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Example Case: Ideal Distillation===

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the haeviest product, heptane, and then separates pentane from hexane in the second column. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. Also, it is a good idea to look up relative volatilites, to further verify near-ideality of the mixture, but also to obtain the information necessary for the Underwood method, which we will employ to obtain a solution. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_{II}(nC5) = 2 mol/s</math>

<math>\mu_I(nC6) + \mu_{II}(nC6) + \mu_{III}(nC6) = 3 mol/s</math>

<math>\mu_{II}(nC7) + \mu_{III}(nC7) = 5 mol/s</math>

<math>\mu_I(nC5) = 99\mu_I(nC6)</math>

<math>\mu_{II}(nC5) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{II}(nC7) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{III}(nC7) = 99\mu_{III}(nC7)</math>

where <math>\mu</math> represents the molar flow, and the subscript represents the product stream.

At this point we have enough information to use Underwood's method to estimate the minimum vapor flows in the column. The following three equations are used in Underwood's method:

<math>\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}f_i = (1-q)F</math>

<math>(R_{min}+1)D = \sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}d_i = V_{min}</math>

<math>\bar R_{min}B = -\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}b_i = \bar V_{min}</math>

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [7].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.

Separation processes

2014-02-20T04:44:52Z

Jxamplas: /* Example Case: Ideal Distillation */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Example Case: Ideal Distillation===

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the haeviest product, heptane, and then separates pentane from hexane in the second column. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. Also, it is a good idea to look up relative volatilites, to further verify near-ideality of the mixture, but also to obtain the information necessary for the Underwood method, which we will employ to obtain a solution. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_{II}(nC5) = 2 mol/s</math>

<math>\mu_I(nC6) + \mu_{II}(nC6) + \mu_{III}(nC6) = 3 mol/s</math>

<math>\mu_{II}(nC7) + \mu_{III}(nC7) = 5 mol/s</math>

<math>\mu_I(nC5) = 99\mu_I(nC6)</math>

<math>\mu_{II}(nC5) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{II}(nC7) = (5/990)\mu_{II}(nC6)</math>

<math>\mu_{III}(nC7) = 99\mu_{III}(nC7)</math>

where <math>\mu</math> represents the molar flow, and the subscript represents the product stream.

At this point we have enough information to use Underwood's method to estimate the minimum vapor flows in the column. The following three equations are used in Underwood's method:

<math>\sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}f_i = (1-q)F</math>

<math>(R_{min}+1)D = \sum_i \frac{\alpha_{ik}}{\alpha_{ik}-\phi}d_i = V_{min}</math>

<

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [7].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.

Separation processes

2014-02-20T04:13:52Z

Jxamplas: /* Example Case: Ideal Distillation */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Example Case: Ideal Distillation===

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the haeviest product, heptane, and then separates pentane from hexane in the second column. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_II(nC6) = 2 mol/s</math>

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [7].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.

Separation processes

2014-02-20T04:12:54Z

Jxamplas: /* Example Case: Ideal Distillation */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Example Case: Ideal Distillation===

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the haeviest product, heptane, and then separates pentane from hexane in the second column. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_II(nC6) = 2 mol/s</math>

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [7].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.

Separation processes

2014-02-20T04:11:43Z

Jxamplas: /* Column Sizing */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Example Case: Ideal Distillation===

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the haeviest product, heptane, and then separates pentane from hexane in the second column. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_II(nC6) = 2 mol/s<math>

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [7].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.

Separation processes

2014-02-20T04:11:21Z

Jxamplas:

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

===Example Case: Ideal Distillation

Assume an equimolar mixture flowing at 10 mol/s of 20 mol% n-pentane, 30 mol% n-hexane, and 50 mol% n-heptane. Separate the mixture into 3 products: 99% pure n-pentane, 99% pure n-hexane, 99% n-heptane. Assume the feed and products are all liquids at the bubble points. There are two process alternatives to consider in this example. The direct sequence removes the most volatile species, pentane, in the first column, and then separates hexane and heptane in the second column. The indirect sequence separates the haeviest product, heptane, and then separates pentane from hexane in the second column. Next, we must decide if these species exhibit fairly ideal behavior during distillation. Since the n-alkanes have very similar properties, it is safe to assume they will display close to ideal behavior. The next step is to look up the boiling points of the 3 species. In this case, the normal boiling points of pentane, hexane, and heptane are 309 K, 342 K, and 372 K, respectively. The next step is to write out material balances based on molar flows and the design specifications. They go as follows:

<math>\mu_I(nC5) + \mu_II(nC6) = 2 mol/s<math>

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

An example of stripping in industry is the deodorization of food items such as oils. The oil is heated and allowed to trickle down the column while steam flows up from the bottom of the column. At the vapor-liquid interface, volatile components of the oil transfer to the steam and are carried off the top of the column, leaving a purified oil product [5?].

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.
# Stripping Column. Alfa Laval Web site. Available at: http://www.alfalaval.com/solution-finder/products/soft-column/Documents/Stripping%20Column.pdf. Accessed February 19, 2014.

Separation processes

2014-02-20T02:30:47Z

Jxamplas: /* McCabe-Thiele Diagrams */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Separation processes

2014-02-20T02:21:18Z

Jxamplas: /* Column Sizing */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
====McCabe-Thiele Diagrams====

===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid stream, and <math>\rho_V</math> is the density of the vapor stream.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Separation processes

2014-02-20T02:20:16Z

Jxamplas: /* Column Sizing */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
====McCabe-Thiele Diagrams====

===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters, <math>\rho_L</math> is the density of the liquid, and <math>\rho_V</math> is the density of the vapor.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Separation processes

2014-02-11T19:25:43Z

Jxamplas: /* Settling and Sedimentation */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
====McCabe-Thiele Diagrams====

===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating agent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Separation processes

2014-02-10T05:16:29Z

Jxamplas:

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
====McCabe-Thiele Diagrams====

===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process [4].

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent [4].

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery [4].

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material [4].

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications [4].

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating gent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal [4].

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid [4].

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities [4].

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating [4].

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy [4].

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium [4].

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Separation processes

2014-02-10T05:14:45Z

Jxamplas: /* Extraction */

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
====McCabe-Thiele Diagrams====

===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process.[4]

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent.

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery.

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material.

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications.

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating gent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal.

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid.

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities.

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating.

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy.

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium.

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Separation processes

2014-02-10T05:13:19Z

Jxamplas:

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

==Introduction==
Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

[[File:Sepmeth.JPG|frame|Figure 1. Separation methods by property]]

==Theory==
===Vapor-Liquid Equilibrium===
Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

<math>T_{liquid} = T_{vapor}</math>

<math>p_{liquid} = p_{vapor}</math>

<math>chemical potential_{liquid} = chemical potential_{vapor}</math>

==Distillation==
===Flash Distillation===
Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure 2 shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let-down valve where the two-phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents, respectively.

[[File:Flash.gif|center|frame|Figure 2. Flash Distillation Flow Diagram]]

===Column Distillation===
Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

[[File:column.jpg|250px|center|]]
====McCabe-Thiele Diagrams====

===Stages===
Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.
====Sieve Trays====
The simplest and least expensive tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

[[File:sieve.jpg|200px|center|]]

====Bubble-Cap Trays====
Bubble-cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble-cap trays require larger tray spacing than sieve tray design. Bubble-cap trays have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

====Flow Patterns====
Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

===Column Sizing===
Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity,

<math>\hat u_v = (-0.171l_t^2 + 0.27l_t - 0.047){\frac{\rho_L - \rho_v}{\rho_v}}^{1/2}</math>

where <math>l_t</math> is the plate spacing in meters.

Column diameter, <math>D_c</math>, can then be estimated using the relation,

<math>D_c = \sqrt{\frac{4\hat{V_w}}{\pi\rho_v\hat{u_v}}}</math>

where <math>\hat{V_w}</math> is the maximum vapor rate in kg/s [2].

==Absorption==
An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure.

The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. The absorption factor of component i is

<math>A_i=L/K_iV</math>

where <math>L</math> is the liquid flow rate entering the column, <math>V</math> is the vapor flow rate entering the column, and <math>K_i</math> is the vapor/liquid equilibrium ratio for component i [4]. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorption factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

==Stripping==
This process separates solutes from solvents (often after absorption, to purify the solvent so that it can be recycled to an absorber). Stripping will depend on the vapor and liquid flow rates, as well as the temperature and pressure of the column. There is a temperature drop down the column, so columns generally have either an increased operating temperature or decreased operating pressure.

The stripping factor of component i is

<math>S_i=K_iV/L</math>

where <math>K_i</math> is the vapor/liquid equilibrium ratio, <math>V</math> is the vapor flow rate entering the column, and <math>L</math> is the liquid flow rate entering the column, will determine how much of solute i will be stripped from the liquid into the vapor phase [4]. The usual range for the stripping factor is between 1.2 and 2.0, with a stripping factor of 1.4 being most economic.

==Other Separation Processes==
===Extraction===
Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process.

===Crystallization===
This process recovers solutes that have been dissolved in solution. The resulting product is in the solid phase. Depending on the material properties of the solute and solvent, the solute is recovered by precipitation after cooling, removal of solvent, or adding precipitating agents. Crystallizers are designed based on phase equilibria, solubilities, rates and amounts of nuclei generated, and rates of crystal growth. Every crystallization process is a unique system, so plant evaluation is usually required before complete implementation. Crystallization can be performed in both batch and continuous processes, and design features can control crystal size to an extent.

===Membrane Separation===
This separations process uses selectively permeable membranes to separate components in a mixture. Typically, one of the components will freely pass through the barrier while the other components will not. The stream that passes through the membrane is the permeate and the stream that does not pass is the retentate. The driving force behind this separation is a pressure gradient. Membrane separation is beneficial because it can separate mixtures at the molecular and small particle level. Furthermore, there is no phase change required so the energy input is low. Limitations of this process include achieving high product purity, incompatibility with certain stream components, low operating temperature, and low flow rates. Although membrane separation is generally not scaled up, examples of scaled-up membrane separation include seawater desalination and hydrogen recovery.

===Adsorption===
Adsorption involves an adsorbent and adsorbate. The adsorbent is typically a solid, and will typically separate the adsorbate from the stream. This process usually includes a desorption step that regenerates the adsorbent for further use. Raising the temperature or increasing the concentration of the adsorbate can reverse the adsorption process. Although the recycle of the adsorbent is a very economic design feature, the downside of this step is that it results in a cyclic process, which introduces complexity to the overall process. Industrial applications of this process are for bulk separations and gas purification. The adsorption/desorption process in these situations involves a large amount of heat transfer, which design engineers must take into account when sizing and selecting equipment material.

===External Field/Gradient Separation===
These separations use external force fields or temperature gradients to separate responsive molecules or ions. The use of these processes is fairly limited to a few specialized industrial applications.

===Settling and Sedimentation===
In settling processes, solid particles or liquid drops are separated from a stream by gravity. The stream can be in either the liquid or gas phase. For vapor-liquid mixtures, flash drums are generally used to separate the mixture. The velocity of the vapor must be less than the settling velocity of the liquid drops for this separation to occur. For liquid-liquid separation, the horizontal velocity of the fluid must be low enough to allow the low-density droplets to rise to the interface and the high-density droplets to move away from the interface and coalesce. In sedimentation, the result of the process is a more concentrated slurry. Typically a flocculating gent is used to aid in the settling process. One way to perform this separation is to use a cone-shaped tank with a slowly revolving rake that scrapes and moves the thickened slurry to the center of the cone for removal.

===Flotation===
Flotation is a process designed for specific solid-solid mixtures. It works by generating gas bubbles in a liquid that attach to selected solid particle. Afterwards, the particles rise to the liquid surface where they are removed by an overflow weir or mechanical scraper. The separation depends on the surface properties of the particles and its preference to attach to the gas bubbles. To meet the necessary requirements of the flotation process, a number of additives can be used to control things like the pH of the liquid-solid mixture, the activity of the solid surface, and the froth that can assist in separation. The bubbles can be produced by gaseous dispersion, dissolution, or electrolysis of the liquid.

===Centrifugation===
This process is similar to external field separation in that an external force field is applied to separate a mixture. When gravity separation is too slow due to particle densities, particle size, settling velocity, or the formation of an emulsion, centrifugation is commonly used. Centrifugal force increases the total force acting on the particle and results in faster separation times. This process is generally used to separate solids from liquids, however it can also be used to separate two liquids with very different densities.

===Drying===
Drying is performed to remove liquid from a liquid-solid mixture and produce a dry solid. Water is most often the liquid removed, but organic liquids are removed from solids on occasion as well. The heat required to vaporize the liquid is usually obtained by a series of gas-solid contacting devices. Feed condition and temperature sensitivity of the solid dictate the type of contacting device that is used. There are two groups of dryers that differ by the dependence of either mechanical means or fluid motion for gas solid contact. Another feature of dryers is to use either direct (hot gas) or indirect (conductive surface) heating.

===Evaporation===
Evaporators separate solvents from a solution by evaporation. The difference between evaporation and distillation is that evaporation requires the solute be nonvolatile. Because of this, a high separation can be achieved with one stage. Evaporators are essentially reboilers, so evaporation is a very energy-intensive process with a high thermal economy.

===Filtration===
Filtration is a process that separates a mixture of solid in a liquid or gas by passing the mixture through a porous medium in which the particles do not pass. Filtration is done by either cake filtration (particles found on the surface of the filter) or depth filtration (particles found within the filter). Cake filtration is generally performed with a cloth as the filtration medium.

==References==
# Wankat, P.C. (2012). ''Separation Process Engineering.'' Upper Saddle River: Prentice-Hall.
# Towler, G.P. and Sinnot, R. (2012). ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.''Elsevier.
# Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). ''Systematic Methods of Chemical Process Design.'' Upper Saddle River: Prentice-Hall.
# Peters, M.S. and Timmerhaus, K.D. (2003). ''Plant Design and Economics for Chemical Engineers, 5th Edition.'' New York: McGraw-Hill.
# Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). ''Process Design Principles: Synthesis, Analysis, and Evaluation.'' New York: Wiley.
# Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). ''Analysis, Synthesis, and Design of Chemical Processes'' Upper Saddle River: Prentice-Hall.

Separation processes

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Jxamplas: /* Evaporation */

Separation processes

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Jxamplas: /* Drying */

Separation processes

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Jxamplas: /* Centrifugation */

Separation processes

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Jxamplas: /* Flotation */

Separation processes

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Jxamplas: /* Settling and Sedimentation */

Separation processes

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Jxamplas: /* External Field/Gradient Separation */

Separation processes

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Jxamplas: /* Gradient Separation */

Separation processes

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Jxamplas: /* Adsorption */

Separation processes

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Jxamplas: /* Absorption */

Separation processes

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Jxamplas: /* Membrane Separation */

Separation processes

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Jxamplas: /* Crystallization */

Separation processes

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Jxamplas: /* Gradient Separation */

Separation processes

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Jxamplas: /* Gradient Separation */